I agree with their decision. It’s a drag for the folks who do it for fun, but we don’t live in an innocent world anymore. Too big of a film event. Batman drew in the mentally disturbed, as did the recent zombie con. If I had to guess, zombie con probably sealed the deal. Everyone dressed in blood and injuries made it extremely difficult to sort reality from cosplay.
In spite of the name, Zombie con was a large outdoor event though. That’s a lot harder to control. (And as the shooter hasn’t been caught yet, the motive is unclear.) Theaters could search patrons or make them walk through metal detectors, but that’s an expense they probably don’t want. Even Disney lets costumers into their parks during Halloween events.