Legal Argument
First, from a legal standpoint, this is not just a "free speech" issue. If these companies had said "we are a left wing platform, and we only allow left wing content, and if you are a conservative you will be shut down," then I would be more sympathetic to the legal argument. That is not what has happened.
These companies gained users and content providers over many years by adhering to a neutral political policy. Untold individuals and firms, such as PragerU, built business models around these stated policies and were led to believe the platforms would carry their content if they abided by these policies which only banned content based on "profanity, nudity, or mature content." As Ben Shapiro demonstrates in this article, and as PragerU demonstrates in its lawsuit, there is overwhelming evidence that these platforms have disproportionately suppressed conservative content that in no way violates its stated policies. Big Tech's actions in this regard certainly could be construed as a breach of contract if not outright fraud - a massive bait and switch that has caused significant business and monetary damage to a major portion of its customer base.
Further within the legal sphere, some are rightfully arguing that offering customer data to benefit the 2012 Obama campaign was a violation of federal campaign finance laws. Not only that, but given the ever blurring lines between large corporations and government, how do we know such an in-kind contribution to Obama's campaign was not made with some future quid pro quo in mind? In this case, where a company is acting as an appendage of a government organization, would not the free speech argument then come back into play?
And, what about the "Christian baker" argument? Recall that the left believes in forcing businesses to serve customers against the owner's consent. Why should we politically accede to a double standard in this case? As long as we advocate freedom of trade publicly, pro-freedom advocates have the moral right to legally challenge enforcement based on double standards. Ayn Rand made just this argument in her essay, "A Fairness Doctrine for Education," in her book Philosophy: Who Needs It.
Given this and various other privacy concerns (e.g., Facebook admitted it monitors apps like their Messenger service where users have a reasonable expectation of privacy), and given the size, reach, and importance of these companies, I hold that it was perfectly appropriate to hold a public Congressional hearing to air these concerns and better inform legislators as to the nature and scope of these problems.
Moral Argument
However, focusing only on these legal arguments disregards the far more important moral issues at stake and neglects the larger political context of our culture today.
These companies are taking action to suppress pro-freedom content which undermines supporters of the American Bill of Rights! They are part and parcel of a political movement, born in modern universities, that seeks to silence speech that its left-wing proponents deem "offensive,", i.e., violently quash anyone who disagrees with them, particularly conservatives and libertarians who argue for free speech, gun rights, and the American Constitution! Keep in mind, these companies are not corner grocery stores but are among the largest corporations in the world that control a vast majority of the planet's front end access to the Internet, and they are actively promoting an objectively evil ideology while silencing dissenters.
Note the terms and language these companies use when attempting to justify their crackdown on pro-freedom advocates. Facebook recently shut down Diamond and Silk, black female Trump supporters, on the grounds that they are "unsafe!" The insinuation that pro-freedom advocates are "unsafe" or "dangerous" is not a coincidence, but an essential tenet of the modern left in their efforts to crush liberty.
Big Tech has been particularly insidious as they have carried out these attacks without ever pointing their banned customers to exact violations of the stated policy. In other words, under the guise of a neutral policy, they are declaring pro-freedom content to be inherently "dangerous" or "unsafe." Such a policy on behalf of the largest curators of information in the world has the effect of normalizing the idea that non-progressive content is dangerous. By suppressing pro-freedom content under the guise of a neutral policy, Big Tech is purposely conflating pro-freedom content with the concept of danger, which has the effect of ripening the culture to this notion and further serving the movement that has inculcated a generation of students to be anti-free speech.
The conflation of violence with pro-freedom dissent underlies the modern campus notion of "safe space" and the attempts to shout down or ban conservative speakers. In this New York Times op-ed, Professor Lisa Feldman Barret went so far as to argue that speech itself can be a form of violence. This ideology manifests in curricula and policies that promote critical race theory, a racist theory that criticizes focusing on colorblindness and posits that all white people are guilty of an incurable Original Sin - "whiteness" for which the only cure is confession, repentance, and likely cash payments. Part of this theory are the concepts of "microagression" and "white privilege" which, again, seek to conflate speech with violence, discussed here. If you have any doubt these companies are on the vanguard of this intellectual movement, consider Google's firing of James Damore or this recent article which documents how Google instructed managers that "individual achievement" and "objectivity" were examples of "white dominant culture."
Now, if we had a largely free society, and some little company somewhere decided to try and amend its policy to suppress its political opponents, it would largely be merely a legal or contractual issue. However, in the modern political context, where the academic left is engaged in a political struggle to overthrow the Bill of Rights, by attacking free speech, promoting "speech codes" and lobbying to repeal the 2nd Amendment, where all of Europe and Canada have succumbed to legal speech restrictions and dissenters are being jailed, and where we are already seeing legal attempts to censor content (like this bill to require "fact-checking" of websites), we should regard attempts by major corporations to aid and abet this cause to be morally despicable and a threat to our country!
Big Tech Hearing Likened to HUAC
The current scenario could be likened, at least in principle, to the late 1940's when Ayn Rand was one of the friendly witnesses who cooperated with the HUAC committee investigating communist infiltration in the United States. At that point in history, communism was a mortal threat to the American Republic as communists sought to spread their evil throughout the world and had state sponsorship in the Soviet Union and throughout Asia. The 1930's were known as America's "red decade," and, as a Russian emigre, Rand understood the threat posed by communism first-hand as she observed the spread of communist propaganda throughout Hollywood. Although when "[a]sked years later about the hearings, Rand said that they were a "dubious undertaking," "futile," and "nothing but disappointments" she regarded the purpose and goals of the committee to be legitimate.
She did believe, however, that it was acceptable for the committee to ask people whether they had joined the Communist Party, because the Party supported the use of violence and other criminal activities to achieve its political goals, and investigating possible criminal activities was an appropriate role of government. "I certainly don't think it's any kind of interference with anybody's rights or freedom of speech," she said.Here we can identify a core principle with respect to free speech. One can say he advocates the overthrow of the government without legal repercussion, but taking action towards that end puts him in legal jeopardy. Similarly, one could say that they'd like to commit a crime, but if you take action towards that end by plotting with conspirators you leave the realm of "free speech" and enter the realm of criminal action. Analogously, you cannot acquire weapons in order to overthrow the government and then claim property rights. You cannot swing your arm into someone's face and claim that their face just got in the way of your freedom of movement. There is always a context within which individual rights exist.
So, is it ridiculous to suggest that Big Tech is a threat to America in the same way that communism was a threat? After all, the communist countries had nuclear weapons and large armies. It's true that Big Tech does not have an army, and, in isolation, it seems silly to suggest that Big Tech is likely to unilaterally overthrow America. It's not silly when you consider Big Tech as part of a broader intellectual and political movement in which it serves a major role.
Recall, the communists didn't physically attack America - they first worked with intellectual allies in western universities, media, business and arts, to spread propaganda in an effort to ripen the culture and create sympathy for communist political policies - and it worked (see the 1930s)! So, the better question is how is a political movement that seeks to end freedom of speech, suppress dissent, and disarm the populace functionally, legally, or morally different than any other political organization that seeks to overthrow individual rights in America? The two main criteria for posing an actual threat to undermine or even overthrow America or western precepts of liberty and individual rights, are ideology and means, both of which, like the old communists, are possessed by Big Tech and its allies today.
Conclusion
Whether left wing political movements gain power by force or by vote, the end result will be the same - dictatorship. Today, there is a real political movement seeking to overthrow the American system that has already had a massive affect on our culture, institutions and legal policies and is actually being implemented throughout the west in once free countries like Britain and Canada. To the extent that these companies use their resources to aid and abet this cause, they are actively seeking to undermine individual rights in this country. While the legal principles and strategy involved could be debated, at best, Big Tech is engaged in a morally despicable cause that should provoke outrage among advocates of individual rights.