While those are good reasons, a suspension is a punishment and will go on the kid’s record. That is a reason to not suspend him. You do not punish good behavior you punish bad behavior.
1) None of us knows all the facts. There are three sides to every story. His side, the gunman's and the truth. Three days will let that come out.
2) If the evidence shows that he did nothing wrong the suspension will be lifted/erased from his record.
3) As I said Three days is a really good cooling off period. If friends or family is looking for retribution this will let the dust settle.