I think the difference, needsajet, is that most of those pools have giant liability insurance policies, and full time lifeguards that can enforce "no diving", and especially "no using the competition blocks" rules. They are vigilant, and constantly there. That may not stop someone from doing it and hurting themselves, but that's what the liability insurance is for. I notice most of those pools remove the competition blocks and store them when there is not an immediate need for them too.
In this case, you have a private pool that is installing a 3 foot high object that can be easily jumped from. There is no other need for that landscape feature (unlike the competition blocks in a "public" competition pool), and I could easily see a legal argument being made that the homeowner was negligent for installing such a thing in the first place in a pool not designed for diving.
It's just one of those things that you can certainly do, but moving past the legal and financial liability, how crappy would you feel if you put in a rock meant for jumping, some kids came over, and one paralyzed themselves for life? This isn't "jumping into the pool for backstroke" from the side, this is willfully adding three feet to the potential jumping height of a 5' deep pool.