He is a gamer and did it to impress his (online) friends, probably. That's what they do in that kind of community.
Well, that is going to be a costly, costly mistake. It is little odd, because getting those clearances should be extremely hard. I know USA cops get a lot of crap (rightfully in many cases), but they stress test sh1t out of ya to become a police officer. Crazy interviews, lie detectors etc...
That kid probably heard dozen of times that he is going to prison for life if any of classified documents are leaked.
By the way, is it even confirmed that he had right access to those documents or did he obtain it through hacking/sh1t RBAC policies?