If fans throwing water bottles on the field are identified, and are charged with disorderly conduct, I agree. Those charges should be dropped. If fans are charged for spraying the cheerleaders with water, same thing. Apology, reprimand, and no one needs a life long criminal record. But that is not what happened here. He intentionally threw a water bottle into a crowd, from an elevated position. Even at 18 he knows that could injure someone. And it did. It knocked a woman unconscious. That is an intentional criminal act, and is rightly being addressed by the criminal justice system.
We can argue about justice vs mercy all we want, but that is a topic for Sunday School. This incident is a matter of public safety, and society at large has an interest in deterring this behavior. There should be consequences for this act.