This is definitely not a crime in the US per the US Supreme Court. Several additional conditions not in evidence are required for speech of this type to fall outside of First Amendment protections.
The Something Awful Forums got investigated by the FBI after one or two people posted threats about the current president.
So, uh, yes. It's definitely something that the federal authorities take a dim view on.
https://www.law.cornell.edu/uscode/text/18/373
> Several additional conditions not in evidence are required for speech of this type to fall outside of First Amendment protections.
Perhaps your point would be clearer if you indicated what specific conditions you believe are missing. Maybe the tweeter had no followers? Idk, I can only vaguely guess at what you're referring to.