Why did they go easy on the Saudis?

A preliminary report by the independent commission probing the September 11 terrorist strikes includes an interesting and troubling detail.

It turns out that a month before the attacks, a US immigration agent stopped and sent back Mohamed al Qahtani, a Saudi national who may well have been the missing “20th hijacker.” And according to that agent, “he was taking a bit of a risk by refusing al Qahtani entry to the United States because Saudis were generally treated more permissively than other foreign nationals by U.S. border agents.”

So prior to 9/11, it appears, immigration agents risked trouble with their superiors by being overly suspicious of Saudi visitors. Why? I hope the commission finds some answers.