After some research, this is what I've concluded. I am not a lawyer:
From what I could gather, if all you do is punch someone in the mouth one time, break your hand, and wait for the cops to show....you could be charged with assault, but you are not going to be charged with AGGRAVATED assault. In order to be charged with aggravated assault, like Oredigger pointed out, it has to be with a deadly weapon, or with intention to cause 'serious bodily injury' with disregard for human life. Legally, 'serious bodily injury' is not a black eye. It's something that can potentially be life threatening.
Police need witnesses and/or evidence to arrest someone. It's safe to assume he wasn't arrested that night because they obviously didn't have those things at that time.
Will be interesting to find out what really went down that night.
"Is everyone around you an asshole? Chances are it's you who's being the asshole."