Twitter has the right to freedom of expression and freedom of association, and this gives them the perfect right to ban people from their platform if they don't agree with what people say. Just like you have the right to ask people to leave your house or business if you don't agree with what they say. Twitter's rights, and your rights, include banning people from platform/home whose political speech they/you don't agree with - which Twitter didn't even do in this case.

In this case, Marjorie Taylor Green's Twitter account was terminated for breach of contract. She was an adult when she signed a written contract with Twitter and opened her account. According to Twitter, she breached the agreement and they elected to choose a remedy that some people don't like. That's just too bad. She isn't facing arrest or imprisonment as a result of her statements. She can go ahead and sue them in civil court if she disagrees!

< whereby it can do all of those internal-facing "exceptional" things you listed

I'm sorry, I hate to repeat myself, but a corporation cannot levy taxes, cannot jail people, cannot execute someone, cannot declare war, all of which a government can do through a variety of legal means. In fact, the US government has so much power that it can do these things through extra legal means and face no consequences whatsoever.

Twitter does not have this kind of power, and I am rightly suspicious of arguments that attempt to frame the situation otherwise, no matter how elegantly or abstractly presented.