I do not know why THAT pool would do such a thing. Its probably the worst pool on the strip except Casino Royales. Its small, shaded, and populated by pasty skin, midwest mid 50'ish cornfed women(ie not good for eye candy).
Anyway, I've recently stayed at Golden Nugget where there is no such rule...a few beds or whatever are for pay, but most of the lounges are open game.
Stayed at Mandalay Bay last weekend and same thing...everything except cabanas is free.
I've found the overcrowding at pools issue to be highly overblown. Yes if you bring a group of six down to the pool area of any major Vegas hotel at noon on a Saturday, it will be impossible to find accomodations. Well duh.
But more commonly, people are just afraid to toss aside the towels that the previous occupants left there and just assume the lounges are taken. My rule at Vegas pools is that if there are two lounges side by side with no personal items on them, its fair game and the occupants have left.