The fact that the US gets most of oil from sources outside the Middle East is neither here nor there when it comes price.  The US is dependent on oil and much of the world's oil production is centered in the Middle East.  If oil does not flow freely from that area, the price of oil will skyrocket.  If supply dips and demand stays the same, prices will go up.  What is more, Saudi Arabia is unique in so far as it has the ability to increase production so as to stem the tide of rising oil prices.  This is particularly important for the US interests.  Not only do US companies have a sweet heart deal whereby they receive oil at 50 cents below market value from the Saudis (I presume there are limits as to how much they can buy), but the US government is uniquely positioned to influence such Saudi increases in production.  As a result, US oil companies are, through the ties to Washington, better able to speculate on world oil markets than other oil companies.  

Now, more than a few think there is potential for a revolution in Saudi Arabia. Given this, it was argued that developing Iraq's oil potential would be extremely important.  Iraq's oil reserves supposedly rival those of Saudi Arabia and just as important are easily tapped.  (This would not could not happen and would not be allowed to happen with Saddam still in power.) What went unsaid was that a US presence in Iraq would help forestall such a thing from happening.  Indeed, not only would US be strategically placed right next store, but by removing their troops out of Saudi Arabia one of the major sources of instability was gone. Wolfowitz mentioned the importance of the latter in an interview.