Do you believe that if AMC had a policy that said, "Sorry, no firearms allowed in our theaters", that this wouldn't have happened? It's not as if the guy asked permission. Maybe if the policy was in place, an usher could have just said, "Sir? Excuse me, sir?" Oh wait, they don't have ushers any more. Never mind.
I was waiting for someone else to point that out as well. It's not like a sign saying "No spree shooters allowed on premises" would've stopped the guy. Which kind of illustrates the futility of any policy banning real guns or replica guns.
Come to think of it, I can see how someone walking in to a theatre carrying a plastic gun might cause undue panic and concern, so I guess the policy change does make sense. The costume ban, not so much. I mean, if some twisted psychopath dreams up an elaborate scheme like this, we can't necessarily change the world in response to it. This was a random unpredictable incident that would likely never happen again. Which is why banning cosplay seems like a bit of an overreaction.