No one claimed it wasnt. They pondered the notion that the administration may have known about it but did nothing. Its the same concept of the pearl harbor consipiracy (which admittedly has far more circumstantial evidence).
That the government knew an attack was coming but did nothing. Why they would do that in pearl harbor its quite obvious, its no secret FDR wanted to join the war. At the time the US population was staunchly isolationist and needed a pearl harbor to rally the country to support joining the war and conveniently, the main target of the Japanese was the American Carriers that were supposed to be present in pearl, but conveniently were not, if they were the pacific fleet would have been utterly shattered and the beginning of American involvement in the war could have gone very differently.
Again, there is no direct evidence, therefore there is no proof the government "let" pearl harbor happen.
This is the style of consipiracy people are talking about in this thread, not that the US govt actually orchestrated 9/11, but knew it was going to happen and let it, for whatever reason, I mean we invaded 2 countries with little no know resistance from the US populace, because of 9/11. No 9/11 no invasion of Iraq or Afghanistan..period.
But again there is no proof, but in both situations the idea is plausible.
The Gulf of Tonkin incident was also thought to be the same situation.
So was the sinking of an American Ship that sparked the Spanish American war.
Again, no direct evidence has ever been presented that the Government allowed nor had any involvement in these incidents, and I am not saying they did. I am just saying, I wouldnt put it past them.