> 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.
Note that the cases where speech is not legal are significantly more narrow than most people think they are. The category you link to has only 11 pages, and half of them are court cases where the Supreme Court decided against limiting free speech. (In particular, the famous "shouting fire in a crowded theater" quote is from a 1919 case that was overturned in 1969, and shouting fire in a crowded theater is not, in fact, illegal. Inadvisable, probably, and likely to get you banned from that theater for life, but not 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.