Vicki Weaver was dead. Lon Horiuchi directly caused the death. It happened in Idaho. No person is above the law. And then we learn that we are wrong.
Some people are.
Such is an act of Congress. Part of this makes sense. Consider laws regulating the carrying of a concealed weapon. There are a patchwork of such laws, differing in every state. If FBI agents are required by their job descriptions to carry weapons, it would make no sense for them to be subject to arrest in states which did not allow weapons to be carried.