That's because the terrorists are the civilians and the moment you kill them they stop being labeled as terrorists. The civilians are generally brainwashed into being the terrorists or are forced to help the terrorists making them just apart of the terrorist support network. It's the same concept as bombing a manufacturing facility during a time of war. Yes the people working at the manufacturing facility are civilians just working but they are producing products that are helping the enemies war effort. Usually by the time armed forces arrive on the ground of which ever air strike happened all the arms/weapons etc... have been cleaned out and so it becomes easy to claim they were just innocent people. Same concept as the terrorists from pakistan who cross the boarder to do w/e their mission was and than cross back into pakistan thinking that the military won't still take them out. It gets reported in the news/media that it's a drone strike on innocent people in a "allied" nation but it's anything but.