We were just at Beardmore Chevrolet looking at cars---and they have every single building posted. No carry allowed. When we asked one of the salesmen about it, he said that it meant you can't carry in their parking lot either.
I let him know that was only true if every entrance to the parking lot was posted, not merely the entrances to the building. I did say that of course, the business owner had every right to ask people to leave his parking lot---but technically, for it to be automatic, every entrance had to be posted.
We also asked him to inform his manager that not only would we never buy any cars there, we'd be passing the word along to everyone else we knew.