Why does this have so much trouble getting through to you? It's your car, and you have no right whatsoever to put your car on his land. Oh, unless he says you can, of course. He isn't seizing your car; he's doing the equivalent of putting a guard and gate on the lot, and having the guard let some people in, and turn other people away.
That is where the debate should end. The employer cannot have a property claim to the car.