I suspect that their view was that deadly force was only compatible with the 5th Amendment in this setting because al-Awlaki was located, purposefully, in a place where neither the host-state government nor the United States had a plausible opportunity to capture him.
Ben Wittes seconds:
It is not enough to say the words “due process” by way of denouncing the Al Aulaqi strike, as though those words represent a discussion-ending argument. One has to specify what process is due to someone being targeted in a particular circumstance before one concludes that the targeting violates due process. If targeting Al Aulaqi were really an assertion of the power to kill any U.S. citizen anywhere based on his speech, I would find it alarming indeed. But I am, in fact, quite certain that Bobby is correct that it is no such thing.