I think that the costume policy is a knee-jerk reaction, one that will either go away or be refined as they find a groove/get used to the new security measures. Maybe it's not, but either way it's easier to say "no costumes" rather than "no costumes except for the extra-ticket event that's 9 months away".
Either way, this sucks but it's the right thing. I'm just surprised nothing has happened yet, and we're all really lucky for that. I agree, it probably doesn't "do" much, but it'll discourage the people that are dumb/crazy enough to try something like this in the first place.