Bill:

I happen to agree with you on this one (surprise ):

from Bill Moore
I personally felt the argument fell short when they argued that Yemen wasn't a battlefied, so therefore the mission was illegal. Whereever we kill terrorists is a battlefield, it isn't confined to a specific geographical region. It seems ludricous to believe that if a terrorist is conducting operations against the U.S. outside of a designated battlespace we can't kill him.
Michael Lewis, Drones and the Boundaries of the Battlefield (Sep 2011) (18 pages), reaches much the same conclusion:

Conclusion

The Air Missile Manual makes it clear that drones are legitimate weapons platforms whose use is effectively governed by current IHL applicable to aerial bombardment. Like other forms of aircraft they may be lawfully used to target enemy forces, whether specifically identifiable individuals or armed formations, if they comply with IHL’s requirements of proportionality, necessity and distinction.

Because drones are only able to operate effectively in permissive environments, the most significant legal challenges facing their development and employment have been based upon where they may be employed. Attempts to apply the strict geographical restrictions that govern the scope of IHL in internal non-international armed conflicts to all non-international armed conflicts, including transnational armed conflicts, threaten to significantly limit the usefulness of drones.

When IHL’s core principles are considered, it becomes clear that the application of strict geographical limitations on IHL’s scope in the context of transnational armed conflicts cannot be defended. The determination of whether the Tadic threshold for an armed conflict is met on the territory of a non-party to the conflict should have no bearing on whether IHL may be applied to the parties to the conflict. In other words, the fact that there is no local violence occurring in Yemen or Somalia should not be used to provide a sanctuary for non-state actors that are involved in an armed conflict with another state.

The answer for how the boundaries of the battlefield and the scope of IHL’s application can be properly determined is found in neutrality law. This is historically how geographical limitations have been imposed upon IHL’s scope in international armed conflicts. It was applied in the aftermath of the 9/11 attacks, with at least tacit international approval, to the situation involving the United States, al Qaeda and Afghanistan. Its application is checked by the consent of the sovereign states involved, making an escalating spiral of violence less, rather than more, likely. And perhaps most importantly, neutrality law’s application to transnational armed conflicts does not lead to the anomalous results that are produced when strict geographical limitations are applied to transnational armed conflicts in which IHL is read to favor its otherwise most disfavored groups.
The bottom line is that the "Laws of War" (IHL to the law profs) follow the participants, who do not fit into the neat little boxes set up for the regular forces of Westphalian states engaged in conventional warfare.

Some of the assertions made in the name of "IHL" are quite amazing. E.g., Lewis cites this (p.8):

Similarly, Mary Ellen O’Connell has claimed that the shooting down of Admiral Yamamoto’s plane over Bougainville by U.S. fighter aircraft during World War II would today be considered illegal because it occurred ―far from [the] battlefield.[40]

40. O‟Connell, The Choice of Law Against Terrorism, 4 Journal of Nat‟l Security Law & Policy 343, 361 (2010).
She actually did say something akin to that in the 2010 article cited by Lewis:

Dean Koh mentioned a case from World War II in which the U.S. set out to kill a named individual far from actual hostilities when it attacked the plane carrying Japanese General Yamamoto, a reputed planner of the Pearl Harbor attack.[93]

93. Koh, The Obama Administration and International Law, supra note 41.

There are several problems with this interpretation. First, Dean Koh did not refer to remote participation. Moreover, many persons killed and detained have had no connection with Afghanistan. Even respecting those who did, the Yamamoto case was not uncontroversial at the time;[94] today it would be in conflict with the basic treaties that form today‘s law on the use of force, namely the 1945 United Nations Charter and the 1949 Geneva Conventions. These treaties provide little or no right to use military force against individuals far from battlefields.

94. Diane Amann relates that at least one of the participants in that attack, U.S. Supreme Court Justice John Paul Stevens, today has doubts as to whether it was lawful. See Diane Marie Amann, John Paul Stevens, Human Rights Judge, 74 FORDHAM L. REV. 1569, 1582-83 (2006).
Not to degrade Justice Stevens' role as a Pearl Harbor traffic analyst (who saw the Yamamoto "shot down" message after the fact, see Bill Barnhart, John Paul Stevens and the U.S. Navy at War); but why try to present him as "one of the participants in that attack."

Talk about indirect "stolen valor"; factually inaccurate argumentation as to WWII; and with respect to the 1945 United Nations Charter and the 1949 Geneva Conventions, an off the wall legal analysis....

And, a material misrepresentation (whether innocent or intentional ?) of Stevens' position. We have that as stated by Thomas Lee to Jeffrey Toobin, After Stevens (2010):

In April, 1943, a coded message came across Stevens’s desk—“one eagle and two sparrows, or something like that,” he said. Stevens knew the transmission meant that an operation based on intelligence from his station had been a success. American aviators had tracked and shot down the airplane of Admiral Isoroku Yamamoto, who was the architect of the Japanese attack on Pearl Harbor and the leader of Axis forces in Midway.

Stevens was a twenty-three-year-old lieutenant, and the mission, essentially a targeted assassination, troubled him. “Even at the time, it seemed to me kind of strange that you had a mission that was intended to kill a particular individual,” he told me. “And it was an individual who was a friend of some of the Navy officers.” (Before the war, Yamamoto had trained with the U.S. Navy and studied at Harvard.)

Ultimately, Stevens concluded that the operation, which was approved by President Roosevelt, was justified, but the moral complexity of such a killing, even in wartime, stayed with him. “It is a little different than your statistics about so many thousands of highway deaths—that doesn’t mean all that much,” he said. “But if somebody you know is killed, you have an entirely different reaction.” The morality of military action became a lifelong preoccupation.
The need to fact check every assertion by the "IHL Intelligensia" makes for a very time-consuming process - especially when they make those assertions with apparent complete certainty.

Regards

Mike