It wouldn't be the first or last time in your life time, the previous or the next where someone has had disciplinary/criminal record for being in a fight they didn't even want to be in or have happen in the first place. When someone attacks you and you can't do much all else except hit back to make the pain stop, in some other peoples eyes unfortunately, they don't care.
Now, I'm not saying this happened specifically here, but it happens, and we don't really know what happened at all, he could have instigated it, he may not have. People fight all the time with lesser punishment.
Meh, whatever, my opinion is that it's laughable it's even an issue, it's a no brainer - HE STAYS!.