You don't have to let your employees have guns in their cars, parked on your commercial property. You can modify their employment agreement so they have to park elsewhere, or you can fire them if they're somehow dangerous or careless with their firearms.
Prohibiting people from having guns in their cars in a work parking lot is bad. It infringes upon the employees' rights as they travel to and from work.
On residential private property, you can do anything you want.
On commercial private property, the rights of employees and customers who are behaving lawfully must be balanced against the rights of the owner.
“The egg hatched...” “...the egg hatched... and a hundred baby spiders came out...” (blade runner)
“Who are you?” “A friend. I'm here to prevent you from making a mistake.” “You have no idea what I'm doing here, friend.” “In specific terms, no, but I swore an oath to protect the world...” (continuum)
“It's a goal you won't understand until later. Your job is to make sure he doesn't achieve the goal.” (bsg)