The order to shoot looters (and rioters) is common. I believe it was in effect during the LA race riots a few years back.
That particular clip of the tank crushing the car was made in the early days of the war.
One thing we don't know is what the current 'rules of engagement' are.
(old article:
http://www.estripes.com/article.asp?section=104&article=13834&archive=true)
Now, back to the main topic here, yes, the law does very specifically state what is and is not allowed. Lincoln had an 'escape clause' worked in to things in his day. Bush and his 'boys' are only following a long, familiar and shameful path, where they look for every legal loophole.
Remember, the taking of hostages is against all of the laws we are operating under, yet US troops have done just that, in attempts to force those on the 'most wanted' list to turn up. This has worked, yet it is illegal. By doing so, they may have saved thousands of lives...but they broke the law. So, who is right?
Keep in mind, the Bush administration is guilty of war crimes, yet it will never be tried..that whole, we won part. The winner defines history, and writes the rule books.