I think a good analogy might be to call "acceptable speech" a form of "regulation". Traditionally, established large companies are fine with additional regulation because it makes it more costly for small competition to operate and compete with them. Likewise with expecting and making it "the norm" for platforms to police their content and get in to trouble for bad content, they make the operating cost of smaller competitors higher (if those competitors choose to play by the same rules).
Right now we're seeing that the smaller competitors are choosing not to play by the same rules, which I think is rightfully making the established companies nervous. From a company perspective it makes them worry about their market share. And also from a personal perspective, the individuals inside those companies worry that the alternatives will allow movements that they don't like to grow.
I'm just expressing why I find this disturbing; as you seem to too.
In the long run, the market will solve this problem by creating alternatives. It is just a matter of time.
I can still express why I think these companies are taking the wrong actions and believe that they have the right to take those decisions. And that there is sensible alternative to a market economy.
I think this is a terrible way to run society - no company should have the scale or power of these companies, but it turns out that this is what can happen if you run things by a market. To argue that they shouldn't do this is basically arguing against a market economy!