It's debatable.
Imminent exhortations of others to break the law should be illegal.
Past that, it's a slippery slope to pre-arresting people for thought crime.
As abhorrent as some of the language is, it seems... dicey... if the US had simply banned gangster rap, NWA, fuck-the-police style music. And it's a tenuous line from threat to actual violence.
Which is why "credible" is usually the standard for charging. I.e. did you make a threat and take actions to realize the threat?
We’ve all said things we aren’t proud of in the heat of the moment, especially under the influence of mind altering substances. And perhaps he was making an (albeit poor taste) joke? We’ve become far too sensitive these days.
However, just because it's narrowly legal to say something doesn't mean it's a good idea; this kind of thing does tremendous self harm to the speaker's public reputation.