I have to wonder how true that statement is (not yours but Southwest's). I know of no security policy that restricts or limits costumes; airport employees dress up all the time.

However, there has been a backlash from religious Christians regarding Halloween in recent years. For some reason, a kids' night out gathering candy and treats has been associated with devil worship and sacrilege against the Christian Bible. Perhaps I have the wrong impression. At my wife's job, for example, employees were asked not to display any Halloween decorations; however, "harvest" decorations (anything minus jack-o-lanterns, witches, ghosts, goblins, etc.) would be permitted. And the boss admitted that Halloween offended his religious beliefs. My point here is not to judge the policy one way or the other; my point is that it seems that this is a trend in many places. I'm curious what will happen at Christmas.

At any rate, it's pretty easy to blame it on "security." I'm just curious if that's the real reason.