As a frequent pro-gunner, the argument as I see it is more that if you want a gun-free zone that might actually be safer, then you need the whole airport-style rigmarole - metal detectors and armed security at every entrance, everybody entering gets their bodies and bags scanned or searched until you can positively verify that there aren't any guns, carefully designed walls, fences, and doors to make sure that nobody ever gets through anywhere but the official entrances with security, etc.

Putting up a sign saying gun-free zone at a place with no security makes things less safe, since it can only deter people who would never commit a crime anyways.