Can't tell if your question is rhetorical or not, but: Falsely yelling "fire" in a crowded theatre is different because it (1) is actually false, (2) leads to an immediate and unambiguous risk of life, and (3) is not potentially of political importance.

The first is the most important in the US for free-speech issues. For instance, "it's true" is considered an absolute defense in cases of slander/libel, no matter how mean spirited or damaging.