This entire al-Awlaki thing is wrong and extremely dangerous for all US citizens. al-Awlaki was still as US citizen when he was killed (
Was Anwar al-Awlaki still a U.S. citizen? | FP Passport). Now, the ability to kill US citizens “if strong evidence existed that an American was involved in organizing or carrying out terrorist actions against the United States or U.S. interests.” originally came from Bush (
U.S. military teams, intelligence deeply involved in aiding Yemen on strikes), but this was illegal as in 1981, then-President Reagan issued an Executive Order (
Executive Order 12333 - United States Intelligence Activities) which stated that "No person employed by or acting on behalf of the United States Government shall engage in, or conspire to engage in, assassination. However, President Bush did it and Obama continues to do it. The serious problems come in when one takes into account 2 things: the fact that al-Awlaki was still a US citizen and therefore had rights and the fact that the President can now name anyone a terrorist.
The fact that al-Awlaki was still a US citizen at the time of his death means that he still had the rights of an American. The Constitution (
U.S. Constitution - Article 3 Section 3 - The U.S. Constitution Online - USConstitution.net) defines treason as "consist[ing] only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort. No Person shall be convicted of Treason unless on the Testimony of two Witnesses to the same overt Act, or on Confession in open Court." We do not know whether or not al-Awlaki actually did what he was being accused of because the US government is not going to give out any of the evidence. Further, when one realizes that the US government can declare anyone a terrorist (
Rounding Up U.S. Citizens » Counterpunch: Tells the Facts, Names the Names) such as "Anyone who donates money to a charity that turns up on Bush’s [or Obama's] list of 'terrorist' organizations, or who speaks out against the government’s policies could be declared an 'unlawful enemy combatant' and imprisoned indefinitely," this means bad news for US citizens. This means that the government can declare US citizens to be terrorists, not give the information condemning them, not hold a trial to prove them innocent, and send in a drone (or whatever) to kill them. The al-Awlaki assassination is dangerous as it sets a precedent, thus from here on out, any US citizen who the government deems a terrorist and kills, the government can justify the killing by saying that the al-Awlaki assassination sets a precedent and thus it is therefore legal for the US government to kill its own citizens.
EDIT: Also, al-Awlaki cannot lawfully be accused of treason as there must be "the Testimony of two Witnesses to the same overt Act, or on Confession in open Court," according to the Constitution.