The same thing happened to us. When we purchased our wagon, the first thing we tried to do when we got home was to remove the cheesy dealership temporary plate cover. We were able to remove the front, no problem, but when we tried to remove the rear screws, we couldn't get them off the vehicle. We ended up having to cute the license plate cover to get it off because the screws were stuck.
We had to take the vehicle to service for a different issue and while there, questioned the service advisor about the rear screws that wouldn't come off. He inspected them and said that whoever put the dealership advertisement license plate cover on the vehicle used the wrong screws. He said that when the vehicle is delivered to the dealership, the correct screws are not on the vehicle, they are in a plastic bag inside of the vehicle. Lazy sales people will just use whatever screws they can find to attach their advertisement.