Of course you can, and they'll be happy when you buy drinks.
For example, people kept asking about the shuttle bus times, so I went over to the HRH and RPR to get them for this page.
http://www.nogodforme.com/IOA072905.htm
I wasn't officially staying, but the waitresses at the HRH pool had no problem serving me drinks as I tipped them well.
The back entrance to the pool area at the HRH has a gate, so you may have to wait for someone to pass thru before you can get in. Otherwise, you'll have to walk all the way around to the front.
It's one of those things, they don't want the general public (GP) just showing up using the facilities for free, but then again, if people show up to visit the restaurants to spend money, then they're ok with that.
The same thing happens in Vegas. The locals try to show up and hang out at the pools for free, so they have to kick them out. Yet people who visit hotels spending money are welcomed.