I get your point, but as long as they cover ALL entry points, and ALL people coming into the space, this is not a classic "gun free" zone. It has long been my thought (and many others, I suspect) that any place the government wanted to be truly gun free was fine, as long as a) they provided adequate security against crazy people with non-firearms weapons, and b) they ensured that EVERYBODY in the space was equally dis-armed.
Courthouses and secure airport terminals being the general examples, it is this sort of full screening that has usually been too expensive to implement that keeps the "gun free" zone from being an actual gun free zone, and therefore gets people killed.
ETA: Now... the blocks AROUND the stadium, as spectators walk to and from the event... THAT'S a whole other story, and you are exactly right. Free unarmed victim zone for muggings and hold-ups!!