You're ignoring the obvious justification: the strike was targeted at different Al Qaeda officials that he was traveling with: http://www.nytimes.com/2011/10/16/world/middleeast/yemeni-se... ("Airstrikes, believed to have been carried out by American drones, killed at least nine people in southern Yemen on Friday, including a senior official of the regional branch of Al Qaeda and an American, the 17-year-old son of a Qaeda official killed by the United States last month, according to the government and local reports on Saturday.").