For this self-control and self-discipline, people would first and foremost have to know the effect it has on them. And that's the problem: They cannot know that.
College is for many people the first time they can get away from their parents. For the first time without constant supervision. And hence of course the first time they can actually get in touch with the things their parents don't approve of. Whether that's drugs, booze or other "bad" behaviour, one thing is certain: They will indulge. Often to excess.
Ordinarily, I'd agree with you. That's what I always tell my American friends, too. But this study has been done in the Netherlands. The drinking age is 18 (used to be 16 like in Germany, IIRC) and Dutch teenagers, much like any European teenagers are much more independent and self-reliant than their American counterparts. Helicopter parenting, while it does exist, is not very prevalent in Western or Northern Europe.