My theory on hot tubs for adults only comes from my experiences with kids in hot tubs... A hot tub is a place for people to relax, and NOT for kids to use as a splash zone. And the only way to combat the problem of immature kids in a hot tub is just ban all kids! Yes, it does negatively affect the kids who can behave in a hot tub, but this is a solution that will make most people happy who don't want to deal with the annoying, splashing kids in a hot tub.
I agree with you on the need for better behavior from some kids. But there's no need for overkill. That's like banning all alcohol because some people who drink alcohol throw up. Better to just hold people accountable for their behavior (or in the case kids, require parental supervision).
The thing about adults wanting to relax could be said for nice restaurants, and Disney doesn't ban kids there, nor does Disney ban kids from any other hot tubs at the hotel pools on property. I would sometimes prefer a hot tub with no kids, but I don't expect to find one at the Poly, Contemporary, Yacht Club, Coronado Springs, or Carribbean Beach resorts.