In most cases it's not "while on business property" that is the problem. It's that stop at the stop and rob on the way home, or walking from your car to a restaurant, a movie, or even your apartment. By not being able to have a firearm in your privately owned vehicle, you are disarmed at all those times as well.
My employer recently changed it's policies, as written rather than as enforced, to not preclude having a firearm in your POV. Unfortunately where I actually work is not on my employer's property, and there they not only ban privately owned weapons, they do spot searches of vehicles to enforce that rule.
See #41