This isn't really any different from the President authorizing the Air Force to shoot down a civilian airliner in a 9/11-like scenario, in order to prevent mass casualties. If one is legal, the other is also legal.
Another thing, semantics are the last thing you want to rely on for protection from a government sanctioned assassination.
If the government wants it to be legal to assassinate you, they will make it so.
edit: I can't help but appreciate the fact that you and I are arguing over my usage of the word semantics.