Reuters explains the rationale, as put forth by the document:
While capturing al Awlaki would have been preferable, the memo states that "we also understand that an operation by either agency to capture al-Aulaqi in Yemen would be infeasible at this time."The memo, prepared by the Justice Department's Office of Legal Counsel, says that because the U.S. government considered al Awlaki to be an "operational leader" of an "enemy force," it would be legal for the CIA to attack him with a drone "as part of the United States' ongoing non-international armed conflict with al Qaeda," even though he was a U.S. citizen.
The memo also says the killing of al Awlaki by U.S. military forces would be legal under an authorization for the use of U.S. military force approved by Congress following the Sept. 11, 2001, attacks on New York and Washington, D.C.
View the full document below, with the drone memo starting on page 67:
Comment: Even more heinous are the excuses made for killing Anwar al-Awlaki's 16 year-old son, two weeks later. He was also a U.S. citizen. Apparently it was all his dad's fault for getting killed earlier, which made his son come looking for him.