Conspiracy theories as a whole in the US, I believe appear more popular than they actually are simply due to the attention they garner from the media. Only a tiny percentage of the population truly believe these and media (news, tv, movie, internet) has given even the smallest person/opinion a large platform.
As for the 9/11 conspiracy... BS. The theories have debunked time and time again.
Here is one good source.
Debunking 911 Conspiracy Theories and Controlled Demolition Homepage
As for the US simply letting 9/11 occur. It did and it didn't. There were multiple failures on the side of intel and application, but people didn't simply "let" it happen as to say they knew it was going to happen without question. To understand this your have to take a deep look at what happened.
In short, after 6 decades of combating communism/soviet union the threat of terror was not seen as a priority. As such resources for this application were limited. For the few intel/LE officers investigating and collecting intel, all believing the threats they were following were real, they "competed" for what limited resources were available for them to push their case further down the pipe. There was no doubt good intentions, but it did result in a massive failure.