Believe it or not, government regulations don't have to be the solution to every little thing.
See saurik's comment below for more.
Furthermore, because Twitter isn't the government, it's probably incorrect to use "censor" or "censorship" at all. "Discrimination" is the correct word to use if a corporation like Twitter terminated her account for an illegal reason. For example: if Twitter terminated her account because she was a woman or a person of color or a member of a protected class.
https://www.merriam-webster.com/dictionary/censor
That doesn't apply? The common definition of censor is not exclusive to government.
By banning people based on the content of their speech, Twitter is precisely engaging in censorship. Just because they haven't called their department head responsible for handing out bans a "Censor" does not mean their constructive behavior is something new that we don't have a word for!
Furthermore, immediately referencing the Bill of Rights to call Twitter's activities "freedom of expression" is wantonly destructive to any sort of discussion. The US Bill of Rights takes natural rights and narrowly frames them in terms of restrictions on government, because that is likely the best that can be done at its level of abstraction. The scale Twitter operates at is much closer to that of government than that of the People - it is most certainly appropriate to say that Twitter is impinging upon individuals' freedom of speech. If you do not agree that it is a concern, then go ahead and make that argument on its own merits. Narrowly construing definitions so that you can discard concerns does not make for productive understanding.
(Having said that my own feeling here is that Ms. Ignorance-as-a-Service has reaped what she has sown, is of a notoriety that is legible to Twitter, and can easily publish on other platforms - compare with all the unnoteworthy individuals who get the automated banhammers with no appeal. And this will ultimately just be a boon to her, just like any other celebrity feud. But let us not twist our own definitions to assuage our cognitive dissonance!)
It's absolutely appropriate to reference the Bill of Rights. As a collection of individuals, Twitter is legally entitled to both freedom of expression and freedom of association. That said, Twitter also has every right to ban Marjorie Taylor Green for breach of contract.
> The scale Twitter operates at is much closer to that of government than that of the People.
I'm sorry, but this argument is objectively false. You cannot compare the power of Twitter to the nearly unfathomable power of the three branches of the US government. Twitter can't levy taxes. Twitter can't declare war. Twitter can't prosecute you and/or put you in jail. Twitter simply doesn't have these powers.
Furthermore by your simplistic definition, the US Government is also just "a collection of individuals". And while that collection has (conceptually) agreed to be bound by the Bill of Rights, there are clearly greater concepts at play than what has been legally defined - the US Bill of Rights does not have a monopoly on the definition of natural rights!
But talking in this framework is ultimately unproductive - it hinders making judgements on the actual details of the relationships, "contractual" or otherwise. And so discussions of individual freedom must necessarily focus on overall qualitative behavior and not simply condoning anything conforming to some axiomatic framework.
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.
This is the legal analysis of how Twitter can do this, which I understand. But you keep asserting it as if it is some universal truth, without engaging with my actual arguments about the constructive result of such framework.
> a corporation cannot levy taxes, cannot jail people
Except that it can - you can sign a contract agreeing to some schedule of charges (called "taxes") or to have to spend time in a certain place (called "jail").
> cannot execute someone
The only reason you cannot sign a contract that allows for your execution as a consequence of default is because your bare ability to contract has been restricted by the larger government.
You're right, the censorship happened even before that: Twitter's contract that says "you're not allowed to express any of these opinions" is the root of the censorship.