Well they don't really know if someone is an adult or not. Just because they say they are 13 doesn't mean that they really were when they signed up. And 13 is hardly an adult now is it?
No idea if that matters retrospectively in legal terms—it’s seems to me that the main problem was providing service to the kids in the first place—but it’s icky either way. Then again there’s an ickiness to the entire project of pretending casual users’ arcane permissions settings from 15 years ago indicate affirmative consent today…