Honestly, it's much easier to come up with a reasonable conspiracy theory around 9/11 than most people think. There's proof that the FBI and DHS knew that the attacks were being planned (iirc they were warned multiple times by Mossad), so it isn't that much of a stretch to believe that the US intentionally ignored the warnings with the knowledge that such an attack would justify another war in the Middle East.

Ignoring the warnings would be a win-win. You get to go to war for oil if they're real, and if they're fake, then nothing happens and life goes on like it always has.