Before the war US had to buy that oil at market price and all profits belonged to local companies (or to Russian ones, which also operated there). Since war American companies (Exxon Mobil) were granted licenses to extract oil from Iraq oil fields, and now the profits go to them. Not to mention that after Americans pretty much ruined the country, American companies make ton of money on re-building it from ruins. Pretty smart business model: bomb the country, and then charge it money for re-building it :)