The answer is that the picture is rather graphic proof of the one thing anti-war activists are doing their damndest to try to make everyone forget: we were attacked first. See, if people thought about that part it might make them think we actually have a legitimate reason to go to war.Yep. The United States was attacked. By Al Qaeda.
Rather a lot of "imposed forgetting" going on, isn't there?
Trying to justify plunging the Middle East into violent chaos and invading Iraq that way is like a German trying to justify invading Belgium because the IRA blew up the Reichstag.