Actually, such a law answers a necessary question where rights conflict. Property owners should have the absolute right to disallow firearms on their property. The owner of the vehicle wants to exercise his right to carry in his car.
A conflict arises when the owner of both the vehicle and gun works for the property owner who bars firearms. Should he at least be allowed to leave his gun in his car, parked in their parking lot while at work, disarmed according to the property owner? As the courts would likely resolve this conflict in favor of the property owner if the law is silent, it is reasonable for the law not to be silent.
Much like an embassy is considered, under black-letter international law, to be the sovereign soil of the represented country, even though it is physically located where silent law would consider it the sovereign soil of the host nation, the car could be, by black-letter law, can be considered to be an extension of the owner's home, even though silent law would have the property owner's wishes holding sway as to what is or is not allowed on his property.
Hence the need for a law stating that the property owner cannot deny you permission to keep a gun locked in your car when he otherwise permits you and your car on his property.
