Bin Laden was fairly specific about what motivated him in his 1996 fatwa. Some of it was moonshine - the presence of American troops in Muslim "sacred land" was, as D'Souza points out, not the case under any normal interpretation of the phrase. Some of it was not - Western culture is, in fact, making inroads in the Islamic world. The question then becomes what parts of this culture are truly threatening and what parts are not?
It isn't quite as easy a matter as one might think. Planned Parenthood clinics, for example - certainly these might be considered offensive not only to Muslims but to Catholics and Orthodox Jews as well. But so is the broader issue of the emancipation of women. For a Western cultural conservative the latter is defensible and the former is not. That distinction makes no sense at all from the Muslim perspective and it shouldn't be a surprise that their own cultural conservatives reject it.
Parenthetically, I hold, along with Bernard Lewis, that no Islamic society will ever be able to take its rightful place in the modern world until the issue of emancipation of women is resolved. It's throwing away 50% of their resources.
And so the clash of cultures. The real difficulty is that the Left has, in its own collective mind, claimed such issues as the emancipation of women as its own, part of an overall "progressive" program that cherry-picks the attractive and blames the unpleasant on its political opponents. Liberty, for another example, is, in the mind of the Left, its own bellwether issue unless it should actually come to fighting for it, in which case a preference for peace, another issue it has arrogated to itself, comes to the fore. The fighting is blamed on the opponents of the Left and the resulting liberty claimed as the Left's own once the shooting has stopped, with the inevitable counter-claim that were its own policies followed there would have been liberty without fighting.
This is a child's ethos but it is the prevailing sentiment within the Democratic party at the moment. What D'Souza has done is to refuse the Left the convenient out that its precepts may be divorced from the overall package that a Muslim interprets as Western culture. One can see why they're angry; it forces a self-examination that its adherents fear. D'Souza is, from the right, demanding the same thing that Christopher Hitchens is from the left - an honest appraisal of which side fits the Left's ostensible ideals and a demand that they behave accordingly. That is not a welcome message.
What you said.