I sympathize with the argument that social platforms are private and can do business (or not) with whomever they please, the counter argument is that these platforms are "Radical Monopolies" (https://wikitia.com/wiki/Radical_Monopoly) apropos of a recent HN thread on Ivan Illich, where the analogy for the effect of being kicked off a platform is not like Ford declining to sell you a car where you can just go get another one, but rather, cancelling your license in Los Angeles.
It's not 1:1, but if you lose your drivers license in a city like LA or non-coastal state, your ability to participate in society (find work, etc) and your social franchise in society is diminished because the automobile has a radical monopoly in American cities.
Twitter and social platforms like Facebook have definitely become radical monopolies for reputation, where if you have no social media presence, you are excluded socially. (Regarding Facebook, try find a date without an instagram page.) Facebook execs even commented publicly early on that people without Facebook accounts should be treated as suspicious.
That QAnon types are so ridiculous and indefensible is what makes them a great example for discussing how and whether to protect minority views. Twitter does have the right to do what they want, and I'm optimistic that these purges will create demand for the divergent platforms that will replace this first generation of them, but to say this right is simple and natural ignores precedents of radical monopolies that were enabled and sustained by political protection, which seems naive. The non-libertarian case for limiting social platforms ability to purge can be summed up in president Obama's thoughts,when he said, "you didn't build that."
Arguably, social platforms that rely on network effects to become radical monopolies didn't "build that," either.