Generally, I would agree that non-school misconduct should not be punished by the school. However there are some differences here that may come into play. If the school doesn't discipline, then the alternative is to involve the police so possibly the school discipline is a better alternative.

Additionally, because it is a sports team issue, the coach may have broader discretion due to the team and camaraderie issue.

As an editorial, these theories and rules make us subjects. For example by the same reasoning if you receive a DUI (maybe not even convicted), or other criminal activity, you can have your professional occupational license sanctioned. Who cares if your doctor is a drunk as long as not when treating you.