On the other hand, when you make a points advance reservation, Marriott assigns a value to your points essentially treating them as a cash equivalent. They provide you with a confirmation and invoice stating how much you owe them (in points) and when it is due (14 days before the stay) and in return they agree to provide you a room. At this point, your reservation becomes a legally binding agreement to provide a service for the stated price and should no longer be considered part of the award program. If they took away your points to make it impossible to book the room, that would be in their right, but to modify a previously agreed upon contract without agreement from both sides is not legal.