Adam Serwer at Mother Jones: Obama Administration Says President Can Use Lethal Force Against Americans on US Soil. I encourage you to read the whole article, but I do want to highlight this piece of Attorney General Eric Holder's excerpted letter explaining the justification for the administration's position:
The question you have posed is therefore entirely hypothetical, unlikely to occur, and one we hope no president will ever have to confront. It is possible, I suppose, to imagine an extraordinary circumstance in which it would be necessary and appropriate under the Constitution and applicable laws of the United States for the President to authorize the military to use lethal force within the territory of the United States. For example, the president could conceivably have no choice but to authorize the military to use such force if necessary to protect the homeland in the circumstances like a catastrophic attack like the ones suffered on December 7, 1941, and September 11, 2001.It seems...curious...to invoke the bombing of Pearl Harbor as a situation in which drone attacks might be useful, particular given that the question was "about whether Americans could be targeted for lethal strikes on US soil." It was not USians who attacked Peal Harbor. USians of Japanese extraction were, however, detained in internment camps in the wake of the bombing, in one of the nation's many great shames against people of color. It is not reassuring to have that history invoked in defense of the hypothetical use of drones on US soil.