Just curious...
Where is this written? I know I read it somewhere, so maybe the contract?
I wonder if it should be written on the website as a footnote to the pools page? Like it is for potty training needed for the club.
Maybe people should be informed of this earlier in the booking process. I over heard a disgruntled passenger on our recent cruise. His daughter was sitting on the edge of the pool and dangling her feet in, and was wearing a swim diaper. A cm asked him to take his daughter out of the pool. He was quite cross

Daughter screaming. He was shouting something about if he'd known this he would have never come.
Maybe there's a lot of people who don't know these rules before they PIF or even get on board. Definitely NO excuse for breaking rules, but might be considerate to inform people with young children that they won't be allowed in the pools.
It doesn't effect us, as our kids are much older, but when they were 2yrs they loved pools and it would have been very stressful to keep them out, and for us, we would have chosen not to cruise until they were old enough to use them.
There will always be those who think they are above the rules, but maybe there's also a number of parents who would appreciate knowing this rule before they see the contract.