A hotel gets paid by Marriott for points redeemed at the hotel but it is at a much lower rate than cash, so a hotel stands to gain financially from a points booking no show, and loses out by checking in a points booking. I guess in theory the hotel might know it's a nightmare to get no shows to actually pay up and some points is better than no cash, but it is also possible they are doing you a kindness. Hard to know without their confirmation.
I thought that hotels are paid by Marriott for points stays at a variable rate based on the hotels occupancy for those nights, and that it can be as high as rack if the hotel is sold out.