The difference is the due process of war.
A secret Justice Department memo sanctioned the killing of Awlaki, a U.S. citizen who became an al Qaeda propagandist and operational leader.
The document followed a review by senior administration lawyers of the legal issues raised by the lethal targeting of a U.S. citizen. Administration officials told the Post that there was no dissent about the legality of the killing.
With regard to the killing as a counter-terrorism measure, the memo deems, in the words of officials, "due process in war."
Justice memo authorized killing of Al-Awlaki - CBS News
HAMDI v. RUMSFELD
Question
Did the government violate Hamdi's Fifth Amendment right to Due Process by holding him indefinitely, without access to an attorney, based solely on an Executive Branch declaration that he was an "enemy combatant" who fought against the United States? Does the separation of powers doctrine require federal courts to defer to Executive Branch determinations that an American citizen is an "enemy combatant"?
Decision: 6 votes for Hamdi, 3 vote(s) against
Legal provision: Due Process
Yes and no. In an opinion backed by a four-justice plurality and partly joined by two additional justices, Justice Sandra Day O'Connor wrote that although Congress authorized Hamdi's detention, Fifth Amendment due process guarantees give a citizen held in the United States as an enemy combatant the right to contest that detention before a neutral decisionmaker. The plurality rejected the government's argument that the separation-of-powers prevents the judiciary from hearing Hamdi's challenge. Justice David H. Souter, joined by Justice Ruth Bader Ginsburg, concurred with the plurality that Hamdi had the right to challenge in court his status as an enemy combatant. Souter and Ginsburg, however, disagreed with the plurality's view that Congress authorized Hamdi's detention. Justice Antonin Scalia issued a dissent joined by Justice John Paul Stevens. Justice Clarence Thomas dissented separately.
Hamdi v. Rumsfeld | The Oyez Project at IIT Chicago-Kent College of Law
Went through this already in another thread... He was not a DETAINEE.
however he was deemed an enemy combatant.