Thanks for the response. I knew they had added it to AoA but wasn't sure if it was a general resort thing or not.
I've been visiting since 1983 and have stayed at the vast majority of resorts on property. On every visit, without exception, I've always met folks at the pool area that were visiting from other resorts. This was especially true at the All Star Resorts and the resorts along the monorail line. A couple years ago, while at the Wilderness Lodge, I sat speaking to a couple who were there from the Contemporary and had hopped the boat to swim there ahead of their dining reservation. It wasn't that I "think I should be able to use the pool at a resort I'm not paying to stay at." Instead, I didn't look at it any differently than say, heading over to another resort to take a carriage ride, or use their boats, or rent a surrey bike. Those all cost money though so maybe they are bad examples... There are free things to do at each resort that you are welcome to come and participate in. Campfires are one that come to mind. Movies Under the Stars is another. Either way, there are quite a few across property. As such, it wasn't something I realized would be met with any problem. Resort pools are one thing on a very short list of activities that my autistic son tends to find comfort in so it was just a quick thought as something to do that night. I sincerely apologize for raising your dander on the subject or implying that this poor little value resort girl somehow had a right to deluxe accommodations. Not at all what was intended.
The policy is stated on signs at each pool, like this one.
There are limited exceptions in that you may pool hop amongst All - Star Resort pools and Port Orleans pools if you are staying in one of those resorts.
In addition, I believe DVC members get special pool hopping privileges, although I'm not sure what they are.
Disney doesn't mind if you resort hop to eat, shop, look around, sit in the lobby, hunt for pokemons, etc. But they don't let you do it to use pools (or fitness centers, for that matter) at other resorts. As you note, it happens anyways, but it isn't supposed to.
As I understand it, the room key requirement to get into the pools was meant to help enforce this policy and increase safety a bit, as it reduces the chances of a kid or drunk person falling in. This also allows Disney to effectively close the pools when lifeguards aren't present, which seems, sadly, to have resulted in shorter hours at many pools, but that's neither here nor there.
Now I hope you'll permit me a mildly humorous story that could be relevant here, depending upon whether your son's autism manifests the same way my son's (very mild, I should add) does. One of his quirks is that he never ever ever wants to knowingly break a rule. He had a minor panic attack one time when we walked around the pool area at Wilderness Lodge before dinner one night (with no swimsuits on or any intention of going in) when he saw the sign that said not just the "pool" is restricted to guests, but the "pool area". We exited quickly.
If your son wants to enjoy the AKL pool on this trip, I'd inquire at the desk of the resort you are staying at to see if something can be arranged. It isn't beyond hotel CMs to try to make a little extra magic.