> The words on Twitter do sometimes include slander and threats which would make them illegal.
The idea of a nebulous "illegality" is simplistic and wrong, in the US. They might open an individual to civil liability, but they aren't in violation of a criminal code, nor tort law.
> That doesn't mean the action of speaking those words are legal
Speech is always legal, in the US. What you think "legal" means is up for debate.