You're right--I SHOULD have the right to yell "fire" in a crowded theater. Otherwise we'll slip right down the slope to totalitarianism.
Only idealists and militia people believe that the rights granted by the Constitution are god-given and must be preserved regardless of cost. That's a crazy position to hold in the real world.
The question here that would actually be interesting to ask is whether it's the case that IN THIS INSTANCE, security concerns are grave enough to curtail freedom of expression in this way. But arguing that no such restriction is legal or constitutional is simply not borne out by precedent or common sense.