There are many reasons to oppose state censorship, but the people who argue that it’s okay for big tech companies to suppress content for ideological reasons on their platforms oppose it on the ground that it prevents the marketplace of ideas, which they believe maximizes the probability that people will discover the truth and settle on it through rational debate, from working as it’s supposed to. The problem is that, if that’s why you oppose state censorship, and to be clear I think it’s a good reason to oppose it, there is nothing about the state that makes censorship bad when it originates from the government but unproblematic as long as it comes from private companies. Indeed, while it’s true that the government can distort the marketplace of ideas, so can private companies.
For instance, if Google makes it harder to find right-wing content, it stacks the deck in favor of the left, because people are more likely to be only exposed to left-wing content. Similarly, if Twitter hides content by conservatives and/or promotes content by liberals, the former are less likely to be heard. Big tech companies don’t even have to completely suppress right-wing content to distort the marketplace of ideas. Even a pro-liberal/anti-conservative bias that comes way short of the systematic suppression of right-wing content would introduce a distortion. Of course, as long as big tech companies are biased in the relevant way, it doesn’t matter which ideologies are penalized by it, it will distort the marketplace of ideas all the same.
The people who defend big tech companies argue that it’s not the same on the ground that if people are unhappy about what those companies do, they can always stop using them and use other platforms instead, whereas people can’t opt out of the government. If the state bans the expression of certain views and you express them anyway, it can use violence to compel you to stop and imprison you, but Google or Twitter can’t do that. However, since the companies in question are de facto monopolies, it’s not actually true that if you’re unhappy with big tech companies you can just use another platform. Moreover, even if it were true, it would be irrelevant.
First, it’s not always true that, if you’re unhappy with big tech companies, you can always stop using it and use another platform instead. For instance, if you’re a journalist and you want to promote your work, there is simply no viable alternative to Twitter at the moment. No other microblogging platform comes even close to having the number of users Twitter does, and most of the alternatives are hotbeds of extremism, so that anyone who joins them is thereby disqualified in polite company. People who reply that, if Twitter were so bad, a viable alternative would have emerged are just missing the point. Of course, it’s possible that, if Twitter’s behavior were really egregious, it would lose its dominant position to other platforms, but its monopoly power means that it can get away with a lot without endangering that position. Again, for bias to distort the marketplace of ideas, it isn’t necessary that Twitter completely suppress certain viewpoints.