Well that isn't quite right. The original idiom came out of an earlier supreme court ruling that was partially overturned by later rulings on the matter:
> Ultimately, whether it is legal in the United States to falsely shout "fire" in a theater depends on the circumstances in which it is done and the consequences of doing it. The act of shouting "fire" when there are no reasonable grounds for believing one exists is not in itself a crime, and nor would it be rendered a crime merely by having been carried out inside a theatre, crowded or otherwise.
https://en.wikipedia.org/wiki/Shouting_fire_in_a_crowded_the...
Incitement to violence is also permitted as long as it isn't immediate and/or likely. This is known as the Brandenburg Test:
https://en.wikipedia.org/wiki/Brandenburg_v._Ohio
https://en.wikipedia.org/wiki/Imminent_lawless_action
