I agree employers should not be held liable when employees bring firearms with them, but the second part of your statement is ludicrous too. Its my property, I can require you to disarm and you have the right not to work here if you don't want to. It is as simple as that. I will add that I also have the right to require you to bring a firearm to work and if you don't want to, you can go somewhere else too.
But the employer should not be held liable if some nut invades the property and starts shooting under any circumstances.
That could be countered by the fact that your actions create an attractive nuisance. Even if the law were written so that for such a lawsuit to proceed one would have to show that the criminal was likely aware of the restriction, the key point would be not so much to punish companies for restricting firearms, as to make their liability-avoidance lawyers discourage them from doing so.