It was not legal in America to carry the weapon. He was underage, which means it was a crime for him to posses it, and he carried it across state lines, which, again, is illegal. Since he admitted to these two illegalities, he should have been prosecuted federally, as the federal government has jurisdiction over interstate commerce and laws. Crossing state lines makes what he did a federal crime. He was charged locally. It’s OK as your Canadian for not understanding the law, but many Americans including the FBI IGNORED the federal crimes committed, letting him off the hook. The judge explicitly dropped the gun possession charge, even though he really didn’t have a right to do since it was enmeshed in a federal crime, not a state of Wisconsin one.