I don't like it, either, and 24/7 is right on the money with the wardrobe suggestions. Also if you have little ones, a diaper bag with all its hidden pockets can conceal a lot, too. They usually just look and don't really dig. Many a year I've entered and gone straight to the ladies room to move goodies from pockets and sleeves back into my bag. I don't let their policy strongarm me into spending more than I'd planned to on concessions.

As far as the legality/search and seizure issue, I don't know how far that would fly. I imagine there's some sort of implied consent when you purchase a ticket to abide by the policies of the facility. It's not like they're grabbing you off the street and searching you.