"Root cause" theology
Brian O'Connell over at Spartac.us has nailed it beautifully, barring one big caveat that I'll get to in a moment:
...This is typical of the Left. When the terrorists act, it is as a force of nature, like animals responding to stimuli. Blame or responsibility for their actions cannot be assigned to them. It is proper, in this view, to trace back the terrorist response to the most recent Western stimulus, because it is only to the Westerner that responsibility can be assigned or in whom the status of moral actor can be invested.
When the West acts it is responsible, morally and otherwise, for the consequences of its actions, and for others' responses to those actions. This is a one-way street. Non-Westerners are never blamed for Western actions.
Emphasis mine. He's got a point: In the standard narrative of the "anti-war" left, Muslims are exactly as capable of moral choice as animals are. But that's not the whole picture, and I don't believe simple racism is the sole "root cause" of the left's view.
Here's why not: Shortly after the Iraqi elections in January 2005, I recall seeing a lefty (it might've been a commenter at Harry's Place) claim that the elections proved that bin Laden was a great benefactor of humanity, since 9/11 set in motion the chain of events that led to the elections. This is a corollary to standard root-cause theology, less commonly invoked but hardly unknown: If they do evil, we made them do it; but if we do good, they made us do it. If your rhetorical point of the moment says 9/11 was bad, you follow the chain back to the first non-Muslim and blame him; but if you're saying that 9/11 was good, why then the buck stops at bin Laden and he gets all the credit.
So it's really just yet another case of the left inventing a distinct and specially-tailored set of Absolute and Permanent Universal Principles to support each distinct rhetorical point, though this one sure does get re-used a lot more than most.