I'd think the hijackers would refer to them as infidels.

Do you really, truly think that the only motivations in choosing to do an attack against America (heck, picking America as the target in the first place) and picking the WTC and Pentagon as the targets of that attack, was because the attackers were Muslim while the ones being attacked were not? If so, why have they not done similarly to all non-Muslim nations? Why not attack symbols or places of power of religion, rather than economics and the military?

Certainly religion is used as a framing device and recruitment tool; it's a powerful ingroup identifier. Especially when you have people doing the same on the opposite side of your fight.

Piece of advice: just because you see the world in purely Marxist terms, doesn't mean everyone else does.

That's not so much a piece of advice as a snipe at what you perceive to be the dialectic I'm using to interpret this. It seems to me that you didn't say that to enlighten me, but to reduce my status in the eyes of what you (and I) assume is a mostly capitalist readership.