I do not know if this is a bug, an oversight or intended.
Really hard to tell.
I'll see if I can find out anything.
But they do it on gyms. I don't see the difference between gyms and pools. Especially if you take the Windenburg one which is kinda both.
I'd support the argument as feedback regarding design however I don't see any reason for characters to change into a towel automatically at locations where every interaction will immediately result in them being forced to change into another outfit. Even at gyms, they change into everyday or athletic immediately after showering since a new activity is immediately selected at random and, obviously, every activity requires the athletic outfit. At the pool, they interact only with the pool unless you direct them so they'll immediately and always change into swimwear.
If someone wants them to wander in their towels they can use mods to disable autonomous swimming and add the spa lockers but the pool was intended to be a 'family' location and the game always spawns children when the lot is loaded. Alternatively you can create a gym with pools and place the gym equipment underground where they can't be seen or accessed.
