UPDATE: apparently some are discussing the secret legal memo and giving clues to its reasoning. Interestingly, the memo reportedly claims that Awlaki would have had to be captured if feasible. The recent New York Times story, Secret U.S. Memo Made Legal Case to Kill a Citizen, states:
The memorandum is said to declare that in the case of a citizen, it is legally required to capture the militant if feasible — raising a question: was capturing Mr. Awlaki in fact feasible?
It is possible that officials decided last month that it was not feasible to attempt to capture him because of factors like the risk it could pose to American commandos and the diplomatic problems that could arise from putting ground forces on Yemeni soil. Still, the raid on Osama bin Laden’s compound in Pakistan demonstrates that officials have deemed such operations feasible at times.
UPDATE 2: Bruce Ackerman has a good article over on his Foreign Policy blog.