And what if they (Al qaeda) blow up the oil fields? As Rumsfeld said, this is a war unlike previous wars.
There are others.
They wouldn't want to permanently cut off their supply of cash they need to become a unified empire. Besides, oil can be had from other places such as Venezuela, Mexico, North Sea, Texas, etc., at a price, but if the economy goes into recession, the lack of demand will bring the price down anyway. If the economy grows along with high oil prices, alternative energy sources and conservation will mitigate demand and again keep a lid on prices. Either way.