It is, which is why Lindh should be treated as an unlawful combatant, even though he is an American citizen.
However, the last time I checked, we don't currently have a war going on in Yemen. No troops on the ground, nobody firing at our troops. Yes, terrorists use "asymmetrical warfare," and we need to eliminate them before they hit us. But there's something especially repugnant about the US government putting an American citizen on a secret "hit list."
The constitution has a way of dealing with American citizens who have committed treason. I believe this scumbag has committed treasonous acts. But there's a procedure for dealing with treason, and it's NOT the President of the US ordering the death of that person. That's a tactic used by the old Soviet Union and its satellites, North Korea, and many other totalitarian states.
Mark
" . . . the President is authorized to use all necessary and appropriate force against those nations, organizations, or persons he determines planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001, or harbored such organizations or persons, in order to prevent any future acts of international terrorism against the United States by such nations, organizations or persons."
In addition, the preamble stated that the obvious point that " . . . the President has authority under the Constitution to take action to deter and prevent acts of international terrorism against the United States."
When someone is on a foreign battlefield, or supports terrorism or terrorist organizations while outside the US, they are not under US civil jurisdiction and are a legitimate target for military action even if they are a US citizen. Hence bullets and bombs can be used, even to the point of specifically targeting a US citizen. Any US citizen who does not want to take that risk has a simple remedy: do not aid and consort with terrorists.
The problem with treason prosecutions is that there are some unsettled legal issues as to what treason is and what proof is required that would take years to resolve and could unravel a case before a jury. Prosecutors thus prefer better defined criminal laws, especially those passed after 9/11.