"Bush says memo did not spell out attack" -- this is the title of a Chicago Tribune story on the President's response to the release of an August 6, 2001 memo titled "Bin Laden determined to strike inside United States." The NYT quotes Bush as saying "I am satisfied that I never saw any intelligence that indicated there was going to be an attack on America — at a time and a place, an attack."
He is building up a straw man. No one claimed Bush knew terrorists were going to strike America on 9/11. Clarke never said that on Sept 10,2001 an FBI agent burst into the oval office and told the President about the terrorist plot but in response Bush only laughed maniacally and said, "So much the better." Clarke never even claimed that Bush could have prevented 9/11.
What Clarke did claim is that terrorism was not a high enough priority in the administrations pre-9/11 universe; that the administration repeteadly brushed off his warnings that a big terrorist attack was in the works; that his plans for precautions the United States should take were boggled down in endless committee discussions. Clarke admits that even if all his plans were implemented, 9/11 would have most likeley occured anyway. But just because 9/11 was impossible to prevent does not mean that it was OK for the Bush administration to put terrorism on the backburner.