And shouldn't we indict Anwar al-Awlaki?
Al Qaeda seems to "fall between the cracks" between being a criminal organization and a foreign nation at war with us. If we'd declared war, it clearly would have made it legal to hunt bin Laden down, under both US and international law. (Or at least I think so; I'm not a lawyer so if I'm wrong, please correct me politely.) In combat, you're supposed to take prisoner someone who surrenders, but if any soldier has been prosecuted for killing someone who might be armed in the heat of battle, I've not heard of it.
Declaring war on Al Qaeda would also have provided a sound legal basis for imprisoning his followers as prisoners of war -- at least until the war is over. In a conventional war, you sort out people rounded up on a battlefield as civilians or enemy soldiers depending on whether they wear a uniform. In a war in which the enemy doesn't wear a uniform, how should you make the distinction? I'd suggest that anyone who says he's a soldier be treated as such, but be required to say who has authority to surrender, declare the war ended, and hand authority to give him orders to his captors. If someone says he's a civilian, I suppose it's more complicated, but isn't a combatant who poses as a civilian subject to being considered a spy?
But I suppose declaring war on Al Qaeda also would have implied de facto recognition of it as a foreign power, capable of waging war. Is that why we didn't want to do it?