I think they look at the cost of installing a metal detector, employing security, and installing lock boxes for firearms, and just say "forget that, it's too expensive!" I hope that one day we can get all the illegal signs down, and just stick with the law. I hate that state parks, open spaces, and government buildings try to mislead the public and gun owners/carriers with their postings.
Oh, my understanding is that local governments can regulate method of carry, necessitating a CCW if one wants to carry in that building. Maybe we can get Constitutional carry passed and be done with it.