Come on now Iscariot – that’s a bit thin.
Aside from what Dustin pointed out about the ecological disaster that igniting the oil fields would entail, the logic doesn’t hold. Just substitute some other parties in your scenario and you’ll see what I mean.
For example, imagine that the Germans had a problem with the French, and threatened to invade. Further, (this is the outlandish part) imagine the French did not immediately surrender, but instead threatened to burn the Louvre if the Germans came in. If the Germans complained that this violated the rules of war, that would not necessarily imply the Germans had been set to go in to France simply to rob the Louvre of the Mona Lisa and its sundry other treasures. In fact, it wouldn’t even imply that the Germans had the looting of the Louvre as a goal of any sort.
Obviously that’s not exactly the same situation, but it’s illustrative of the point. There are many reasons other than the desire to own the oil that the U.S. would protest the Iraqis blowing up the oil fields.