Their house, THEIR rules - don't like them? Go elsewhere. If you cannot be trusted as an employee to follow company policy, then I would have zero use for you as an employee.
If I cannot trust you to follow the rules without me searching you or your vehicle then why should I employ you?
Character is doing the right thing, even when no one is watching
That said, with a few exceptions, it is illegal in Florida for an employer to prevent a gun locked in a car, but carrying INSIDE the building is easily prohibited