The majority of people who do this know they are 'chancing it' and when caught accept their fate with good humour.
But occassionally I get the odd one who finds it incredulous that if seats are empty they cannot use them. I usually ask what would happen if they went to a BMW dealership and bought and paid for a 3 series car. Upon collection they see a top of the range 7 series car sat unoccupied on the forecourt. Do they suggest to the salesperson that the car isn't been used so I might as well take it instead of my 3 series. What do they believe the response would be.
As I said, this is a rarely used back up tactic. A complementary newspaper of glass or bubbly and a cheeky grin resolves most issues amicably.