The clincher is this: the strongest motivation, according to Pape, is not religion but rather a desire "to compel modern democracies to withdraw military forces from the territory the terrorists view as their homeland."
If you are talking about in Iraq, that is self-evident. If you are talking about the Al-Qaeda terrorists, then they are the stupidest people in the universe, otherwise that thesis makes no sense.
US policy in the Middle East between the first Gulf War and 9/11 consisted almost entirely of disengaging militarily and placing the region on the proverbial back burner. Their string of attacks leading up to 9/11 coincided with the United States keeping Muslims from getting massacred in Bosnia, Kosovo and Macedonia, and some minor diddling with Saddam Hussein over the no-fly zones, a guy who they presumably didn’t even want to remain in power, either. Bush wasn’t an avowed interventionist until after 9/11; in fact, one of his critiques of Clinton was the latter’s tendency to send the military every which way all around the world. If the pre-9/11 trend in US policy toward the Middle East continued, Al-Qaeda et. al. would quite plainly get what they wanted from the US without firing a shot, if what they wanted was "US getting out of the Middle East."
Blowing up US forces where they are is an effective method for getting them to leave, generally, e.g. in Beirut. Blowing up US civilians is an effective method to get US forces to come to you, generally. Had Al-Qaeda really wanted us out of the Middle East, they would have conspired to get rid of Saddam Hussein, who was the only reason we had forces in the region in the first place, and who by the way was an ideological enemy of theirs. Or they would have concentrated on attacking our forces in Saudi Arabia, in the hope of producing a pullout along the lines of what got the US to leave Beirut. Or they could have tried to overthrow the Saudi government.
Conversely, only two things would lead to US military intervention in the Middle East: a) a direct threat to our oil supply, and b) a really effective attack on US soil originating from the region. A) is what caused us to drive Iraq out of Kuwait during the first Gulf War. B) is what Al-Qaeda did on 9/11.
The whole thesis that Al-Qaeda, as a whole, has been motivated by a desire to drive the naughty Americans out of their back yard is extremely silly. Their actions could not be logically expected by anybody to produce the result they allegedly wanted, but rather the opposite. Therefore, their motives must be something completely different.