If I remember correctly, if the hotel is booked (i.e. something like 95% or more) the hotel actually gets reimbursed for the award night very well (something around what everyone else is paying.) When it isn't booked up, the payment is much less, but still more than covers their variable expenses for someone staying in the room.

So between the hotel getting paid fairly well for the room and the penalties they have for walking a Plat, it usually isn't in their best interest to walk a Plat. In fact, unless it is an unusual circumstance (i.e. like someone already mentioned where almost everyone is a Plat), I've heard of employees getting in trouble for walking a Plat (due to the extra cost the hotel incurs from doing so.)