Words uttered in a situational context that renders them of immediate harm are illegal. I can say "Fire!" in a theater while giving a lecture or putting on a show. I cannot knowingly claim the theatre is on fire when it isn't to cause a panic.
Point is, it is not the Word or content that is illegal. It is the union of word and context that is illegal.
Subtle difference, but it's the only thing that keeps that type of law from getting absurd and out of hand very quickly.