Sometimes they require prepayment of the room if you park and fly before staying. Some people have tried to take advantage of it then leave after returning from the flight and not stay at the hotel.
I've had hotels tell me that the rate/reservation become nonrefundable once you have parked there. This could be an issue if a flight delay/cancellation creates a trip in vain or if you are forced to reschedule the return flight. Assuming that you have used a credit card to guarantee the reservation, I don't understand how one could just drive away, thereby parking for free, especially if you call to use the hotel shuttle from the airport.