If someone gets shot or stabbed at that establishment can the victim sue the place for not insuring that all on their property do not in fact have any of the prohibited items?
Businesses have a general duty to protect patrons from foreseeable harm. They don't have a duty to protect you from being the victim of a crime, unless they have some reason to believe that their patrons are the specific targets of the criminal actions. And even then, everyone is a prospective and potential victim.
Just like you, I tend to think that signs like that are meant to be an assurance that I am entering a "sphere of safety." Since they can't even guarantee that the movie won't suck, I don't put a lot of stock in their assurances.
So, their premises lability will pay your medical bills, and that's all that you'll probably get. If you're lucky, they may throw in a free trashcan-sized popcorn.