My wife works in event management and works frequently with contract lawyers so I asked if she could get their take on this. I think Marriott is in a very grey area with this change.

It is very much in Marriott’s right to do as they please with their award program based on the T&Cs. They could remove the program tomorrow and take away everyones points if they wanted. They could take away points as a way to prevent people from booking points advance stays as a way to combat the abuse of the program.

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.

Marriott can put whatever they want in their T&Cs, but they also have to abide by the contract laws which supersede their T&Cs.