I see it as a miscalculation that backfired. Someone decided that we needed access to cheap oil, both from the mid-east and the Caspian, more than we needed to protect our own citizens against terrorism. After all, terrorism can be handled, as is evidenced by the the Homeland Security recommendations made months before the 9/11 attacks. I don't for a minute think that such utilitarian calculations are beyond the brilliant minds that run our foreign policy.
Cui bono?
Saudi Arabia is increasing production, breaking the OPEC limits. Others will do likewise in response. Oil futures are down since the attacks. (We've had no crude supply problems for some time -- the shortages, to the extent they've been real, have been the result of refining capacity shortfalls.)
Maybe it was a foregone conclusion that something like this would eventually happen and rather than focussing attention and resources to preventing it they were focussed on how to respond in the aftermath (domestically, at least.)