You can argue it's wrong or imoral or destructive to society or whatnot, by you can not argue that, within the context of US law, it is illegal. They are perfectly within the bound of the law to do what they do.

What would NOT be lawful would be Trump to close them down because they did what they did. That is full stop illegal if he did it. Not sure how legal it even is to threaten it this way.

At this point, Twitter might as well even close Trump's account. Still legal, still within their rights.