I think because the alcohol enabled the assualts.
Tangent warning… People often overlook the alcohol problem. It really is the core cause of a lot of other crimes because people revert to human nature when drunk. And human nature is really terrible. The real crime should be getting that drunk in the first place without proper saftey protections for others. Because why wait for another crime to happen? Now of course these were teens, so things are a little different, but in general, being an uncontrolled drunk should be a crime. And any crimes committed while drunk should be at least double the normal penalty.