And why doesn't Twitter have the same rights that you outlined in another comment: the right to not have to tolerate a private citizen, the right to prevent someone from saying whatever they want on your private property.
Basically, why doesn't Twitter have the same association and private property rights as Musk?
They do have those rights, which is why they can ban people. The argument is that they've produced a de facto public square, not a legal one, because it's where a massive amount of "public" discourse takes place. Musk is trying to buy them to make their product more consistent with a legal public square.
Personally I think it can't be done without the government getting involved at some point.
If you are a private company absolutist, I guess you could argue that these companies have the same rights to prevent people from saying whatever they want on private property.
Others believe that these platforms are large and powerful enough to warrant a different set of views and regulatory scrutiny.