When is censorship, if ever, justified?

It is illegal in the U.S. to yell "fire" in a crowded theater if there is no fire, for instance. That is censorship. Is it wrong? It is justifiable to lie in order to cause widespread panic and possible injury and death? People cannot even joke about hijacking, etc., in airports. It is illegal to threaten the life of someone. That is all censorship. Is it "unjustified"?