It has to do mostly with the obsession and paranoia Americans have with pedophilia, and a misdirection of their efforts against it.
The fact is that until about 30 years ago, it was fairly common to see prepubescent girls at the beach without a top. Parents then correctly understood that the chest of those girls was no different than that of the boys, and did not have anything that needed to be hidden or covered. I have dozens of pictures of myself and my cousins at the beach when we were little and none of us was wearing a top. Parents would get girls a top when they were beginning to develop, hence, when it was socially appropriate.
Nowadays, they think that baby girls need to wear a top because if not pedophiles will be lusting after them or seeking them. This is absurd. The place where kids are truly at risk of being approached by potential pedophiles is not the beach or the pool, where they are almost always with the parents. It is places where they leave them alone with other adults. Think of church, schools, daycare, at their own homes with older relatives.
However, putting on a top on a girl who doesn't need it gives them the impression they are "doing something". If parents are truly concerned, they have to monitor the environments where they leave their kids and also educating them on reporting predatory behavior from adults. THAT is what really works.