The same reason a 1-14 & 16 & 17 year olds need supervision. They are kids, kids do stupid things, and as a parent your ass is/should be responsible for their actions to at least some extent.
Ok, I might read "supervise" as "babysit". But if your point is if 14-17 yrs old need someone "older" to watch over their every moves, then I'll have to disagree with you.
I don't know how old you are, but I bet when you are that age, your parents advised and hope you take their advises. But if you f*ck up, then I hope they see you are/were old enough at that age to take full responsibilities for your action.