(Unfortunately HN put me in the "You're replying too fast, please slow down" state last night so I was unable to reply, and so I'm not sure if anyone will see this reply now. I wonder if that should be called censorship?)
There is absolutely a "market for censorship," because there's a market for everything a company does! That's the whole idea of the free market - every decision a publicly-traded company makes is a participation in the market, and they take the whole state of the world into account as best as they can. When a corporation says "Happy Pride" or "Merry Christmas" or whatever, sure, part of that is the genuine belief of some of their employees, but there is absolutely a decision that doing so is better for the business than not. (Some of this decision is based on whether it's better for the business to make those employees happy. Most of it is based on whether they'll make potential customers angry by doing so or not doing so.)
If social media companies are getting heat in the public discourse for not doing X, then yes, it's a market-based calculation to say to themselves, "If we keep not doing X, our public image will suffer, which is bad for the business, so let's do it." And since the public discourse changes, the market conditions that lead to certain decisions change with it, too. If on December 1 the media gets mad about people not saying "Merry Christmas," on December 2 the market value of deciding to say it has gone up, and nobody would be surprised to see multiple companies react or expect them to have talked to each other before reacting. If last week the media gets mad about Holocaust-denial content, this week the market value of banning it has gone up.
... And there's a more visible form of this which I left out because I thought it was implied by "responded to market conditions": both companies wanted to make an unpopular decision because they think it will be long-term good for their business, but whoever moves first will suffer a short-term loss. As soon as one company does it, though, that gives cover for any other company. You see this in pretty self-explanatory free markets like competing gas stations across the street: if station A raises their prices, station B is free to raise their prices to match without losing any business relative to status quo ante. So they both end up waiting until they're pretty confident the other wants to raise prices too (which - again - they can judge from the state of the world and not from talking to each other), and then someone updates their signboard, and then the other gas station follows.
It's not collusion when two gas stations raise their prices to the same price. It's how the market works. You can dismiss the fact that the free market is fundamentally an engine of communication as "not interesting" if you like, but that doesn't change how the free market works. It's not coincidental at all - they participate in the same market. But it's also how the market is supposed to work.