Judges and juries don't control what cases they get handed nor write the law. Looking into the details, he actually got off ridiculously light here, because he wasn't just charged with destruction of property, he was also caught in possession of a pair of "high capacity magazines" of some kind (DF defines a high capacity magazine as being anything more then ten rounds, so it could be virtually anything), possession of which carries a 3 year sentence, because it's DC and they're stupid.
Getting a couple months after being convicted of destruction of property and a firearms charge is a very, very lenient sentence and in no way represents some gross miscarriage of justice.