well, to be honest there are certain things that could lead people to believe that

ie the complete lack of intell on it,

the fact that an incident was needed to go out and declare war on muslim extremists etc


think about it this way, in 1939 (or 3 when the reichstage was burnt down, how many people believed it was their own government that did it??
Instead they blamed a group of people - communists, replace the commies with the arabs

et voila!

just another view on it