As an Australian national he can't be a traitor to the US, strictly speaking. He's an asshole, releasing classified material that may serve to undermine allied governments, and we can "persecute" him as much as we'd like on those grounds...though of course the rape charges he faces in Sweden must be pretty damn ugly if he's willing to break bail and hide out in a foreign embassy to avoid extradition.
No treason on his part, as far as I'm aware. I don't think that's the case for that US soldier that passed along a big wad of classified stuff to wikileaks. That guy is guilty as hell, and should be damn glad they aren't pressing for the death penalty.