I think most all the deluxe resort quiet pools have posted hours for closing. It will be a wooden sign near the end of the pool, also listing No Running, No Diving, No glassware, etc.
Again as everyone stated its a courtesy to those guests with rooms nearby to be quiet. Most probably are, some are not as was the case at BCV the night security ran them off.
In fact most late night swimmers are probably so quiet most do not even know they are there.
The night the ones at the Villas were asked to leave, they had the pool side chairs in the pool and yes they were drunk.