The reason this is done is because they are children -- not adults. They are immature and may be making bad decisions. They might have a rotten home life with bad role models. The point is, you try to help them anyway in case they do indeed see a reason to turn their life around.
Why do you think schools have so much sports education? One reason is because it keeps some kids around who otherwise would simply drop out. And every good teacher knows that you can often get a problem kid in your class to focus and learn something by having one or more of their coaches get on their case.
Because:
(A) in high school they're still kids, and
(B) one main goal here is to have an educated citizenry, and this means educating all kids as well as we can, whether the kids themselves think it's a good idea or not. :)