Wasn't a major reason they were pissed off by the US presence in Saudi Arabia that some very major holy sites of their religion are there (such as Mecca), and they objected to having infidels near such sites?
That's probably an oversimplification. The Saudi government's gobsmacking flagrant corruption was a powerful motivator for bin Laden, and religion is a vector for opposition to the Saudi state. And there's no question that the US helps keep the Saudi government in power.
The original stated objection of AlQ was indeed to remove US presence from Muslim holy sites, but Lawrence Wright's book on what happened (for example) makes it pretty clear that the religious stuff was more emblematic of the real problem than it was the real problem itself.
The original stated objection of AlQ was indeed to remove US presence from Muslim holy sites, but Lawrence Wright's book on what happened (for example) makes it pretty clear that the religious stuff was more emblematic of the real problem than it was the real problem itself.