There is a very good book in the 9/11 conspiracy called The Big Bamboozle by Phillip Marshall, a commercial pilot.  The backers were the Saudi Arabian Govt, and VP Cheney was heavily involved, as was Rumsfeld and probably the Bush family.  Reason?  Several.  Increase oil price, excuse to go into Iraq, excuse to gain control of US citizens, $$$$ for US corporations in rebuilding Iraq and other actions.  Bin Laden was just a scapegoat.  There is currently a lawsuit against the Saudi Govt by our Govt that you don't hear about but it IS in the news.  http://www.nytimes.com/2012/03/01/us/graham-and-kerrey-see-possible-saudi-9-11-link.html?_r=0.  These senators have fought hard to find the truth against others in govt trying to cover it up.  Not a theory anymore.