There is no censorship on this site. But there are requirements for what is "in bounds". They don't have to do with content per-se: You are free to discuss 9/11 or The Tuskegee experiment or any other "politically incorrect" question.
The requirements? It must be Solid History: You must document and explain your claims, whether in questions or in answers - back them up with solid historical evidence: Peer reviewed journals; respected journalists and authors with credentials proving they know what they're talking about; clear, incisive analysis of authenticated primary sources; clearly documented forensic evidence; well based statistical analysis; etc. etc. Wikipedia? Only a starting point.
Your own private notions, or undocumented claims, or rumors, hearsay and plain old speculation are not within the bounds of History here. This is not a public opinion blog or fun and games blog or a place to "get out your message", as you have explicitely stated to be your intent in your comments on this answer - not the HuffPost or the DailyBeast or WND.com or MediaMatters or Facebook or Yahoo or whatever - it is a serious site for serious academic work.
Yannis says that perhaps it's boring. I don't think it's boring, but it is challenging. I post only after a good deal of analysis and research which I try to present in my questions and answers, and I am careful to stick to subjects in which I feel fairly well grounded. Still, not everything is so well received. Doing a quick google and/or pulling up something from Wikipedia on the spur of the moment is not enough here.
I have scholarly credentials and extensive background in writing and research - but I am not a historian. To be a contributor on this site if you are not professional historian is challenging work, not necessarily fun. If you're looking for that, this is not the place.
Regarding your 9/11 question:
It can be a valid question, but it must be submitted based on the above mentioned guidelines: You cannot simple cobble together a bunch of rumors and personal theories and expect it to float here. Document your question: Show that it has substance; that is a topic worthy of discussion by serious historians who are not simply interested in "getting off" on a juicy story about some government plot to take over the Middle East or the World or whatever.