But, they have a right to do it. Just like they would have a right to put up metal detectors and hire a **** ton of Barney Fife's to enforce a strict gun ban. It's just not practical do do that.I think the signs do a couple of bad things though.
1 - They might make some people not carry in the mall and therefore reduce their defensive capabilities and safety.
2 - They might encourage a mall shooter to pick this mall believing it is a softer target.
But, in that unlikely hypothetical, I think by strictly enforcing a gun ban in the mall, they're communicating that they're mitigating your risk. I think then they would bear at least a little responsibility if someone got through anyway.