Well, Apple technically has the rights to choose what can and can't be an app, just like Nintendo can choose to not have AO-rated games on their systems.
I don't see why they can't have an age limit confirmation or password re-enter when downloading the app. It should be the parent's responsibility to monitor that stuff. Corporations shouldn't have to be our baby-sitters, and if there's ever a day where parents give that right to them, God help us....