While I think the intelligence agencies were just incompetent to identify the proper threat, perhaps for lack of an adequately warped imagination (911 was a game changer in the rules of plane highjacking - nobody ever used a passenger plane as a missile before), I don't think anyone allowed it to happen on purpose.
However, the president and his associates certainly saw the opportunity and ran with it. The damage done will take decades to undo.
"911 was a game changer in the rules of plane highjacking - nobody ever used a passenger plane as a missile before"
True for civilian planes, but it stretches the imagination a little too far that the US military could discount planes as missiles, given the precedent of pearl harbor.
However, the president and his associates certainly saw the opportunity and ran with it. The damage done will take decades to undo.