If you ignore the fact that people want to take advantage of amenities they didn't pay for, it still comes down to safety (as others have said).
To rebut Epcotdad's comment about "taking his chances in a pool over second hand smoke" - you can move yourself away from smoke and be pretty darn safe. But if you have a child in an overcrowded pool who goes under and can't resurface, and the lifeguard can't get to them because the pool is wall-to-wall people, you might think differently when you're at that child's funeral.
For the same reason that only a certain number of guests can stay in specific hotel rooms, pools are designed for a certain number of occupants. And it's all because of safety.
Buildings - including hotels - are designed for a maximum number of people to reside in them safely, based on a number of factors including flame spread, level of fire protection, response times, etc.
The pools at the Disney resorts are installed in quantities and sizes appropriate to the number of guests likely to use them (based on ratios, historic numbers, etc). I'm sure there are laws regarding the ratio of lifeguards to pool occupants, but I can't cite that.
But if we circle back around to the ethics of it, if I pay $400/night to stay at the Poly, I've bought the right to use their awesome pools. If you only paid $89/night to stay at Pop Century, you only bought the right to use their pool, not mine.
If you want nicer things, you have to spend more money. But that's not the way our society is headed, and this pool issue is just the tip of the iceberg.