If you don't have any rules forbidding firearms locked in cars, why would you have any liability in that scenario in the first place?
I would think that a rule forbidding firearms from vehicles should increase liability in (at least) two ways:
If an employer wants to search every car every time it enters the parking lot, fine.
One, I can't do this.
Two, how would I know what you have in the privacy of your vehicle?
The liability comes into play if something happens while you are on the business property and act improperly to violate the rule. As long as it is locked in your car, it's not an issue. Once it leaves that sanctity, is another matter. The hope is you are responsible.
It all boils down to costs for insurance to keep the business alive. Not having a rule like this increases the premium to an outlandish sum. Look how many companies have this kind of requirement for hiring. the problem they have enforcing it, is invasion of private property.
And try saying you have a right to privacy on a military post as a civilian. See how that works.
I ran a private- no walkin gunsmithing business out of my garage. My premium was higher than the gross I made each month. That was a 60 hour + week. When I ended the business, my insurance was cut in half.