IAAL.,
U.S. Constitution, Article 3, Section 3, says: "Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort."
The constitutional definition of treason was deliberately made very narrow, to prevent prosecutions for anything less than significantly aiding actual attacks by a foreign power. So:
-- With no declaration of war on either side, does planning and aiding terrorist acts constitute "levying War"?
-- Given the informal, non-national structure of al Qaeda, can it be designated an "Enemy"?
These are not trivial questions, but I think it's possible to answer both of them "Yes."
A traitor must be an American citizen -- otherwise he's just a foreign enemy.
The final question is, once we determine that Anwar al-Awlaki is a traitor, may we put out the equivalent of a "Wanted, Dead or Alive" poster?
I think that if we could try to capture a traitor, we can also attack and kill him, just as we could a foreign enemy.