It's an interesting debate.
I mean, everyone should wear what they are comfortable in. (Comments such as "that person should have looked in a mirror" bother me, because, what if they are perfectly happy with how they look, why should they not be because they don't conform to your ideal?)
But, on the other hand, it should be remembered that Disney is a public, and family oriented place. This shouldn't hugely restrict people's clothing choices, but, it might make people think. Such a huge amount of photos are taken at WDW, and I know I would feel uncomfortable if I accidentally had, in the background of my family snap, a potentially underage person showing "part of their body usually covered by swimming costume or underwear" (which is the definition used to teach children which parts constitute inappropriate touching, and includes the buttocks) just because, I would not want to share that picture. Maybe it is the safeguarding training coming out, but, it would make me uncomfortable.