The conspiracy theories generally hold that 9/11 wasn't perpetrated by "terrorists", regardless of their benefactors. E.g. WTC towers were brought down by explosives, not planes crashing into them, Pentagon was hit by a missile, not a plane.
The simplest and most plausible theory is simply that Bush and Cheney knew, but didn't do anything because they needed the attack to go into Iraq. Even then, at least tens of people in the White House and upper echelons of US intelligence would have known and would have known that they could have stopped this by making a few phone calls. Even 12 years later, not one of them has regretted his complicity in killing thousands of civilians and spilled the beans to a reporter.
The simplest and most plausible theory is simply that Bush and Cheney knew, but didn't do anything because they needed the attack to go into Iraq. Even then, at least tens of people in the White House and upper echelons of US intelligence would have known and would have known that they could have stopped this by making a few phone calls. Even 12 years later, not one of them has regretted his complicity in killing thousands of civilians and spilled the beans to a reporter.