Not sure anybody said he was in on a plan to kill thousands of citizens and bring down the iconic NY buildings using planes - thats just a generalised outlandish summary of it to make people who question it look a bit silly (something a conspirer would do )
Most people who i've spoken to about it doubt that he wasn't aware of an attack on US soil. He may have been warned and failed to act, either through being incompetent at his job or he believed an attack would allow a response, 'for the greater good'. Key word being MAY; none of us know what actually happened so can't say one way or the other.
There are rumours that Britain knew about Coventry getting bombed to **** during WW2, but Churchill couldnt deter it without giving away we had broken the Enigma code. I also believe that Churchill sank a French Naval fleet during WW2 to prevent Germany capturing those ships, killing over 1000 French soldiers.
It's not about 'what people want to believe'; some people just question the truth they have been told, and quite rightly so.