Because it's incredibly insulting to Bush, the American government in general, and every single person who died.
There's no disrespect whatsoever to those who died by discussing this stuff, unless you went extreme with it and portrayed them as martyrs on the altar of freedom versus terrorism or some equally grandstanding bullshit that isn't applicable whether you're talking about the theories or real-life events or whatever.
Not really. Annoyance perhaps, but not enough to dissect posts line by line multiple times in a row.
I don't see how making up what amounts to fiction in a book makes anyone suffer. Conjecture never hurt anyone, and it's entertaining to come up with stuff like this. "It's entertaining to think about your own government killing thousands of its own people?" will be the response to this, and yes, it is. Just like it's entertaining to think about how one of our country's greatest leaders was assassinated, how another great leader could have previously known about an attack on a naval base that sent us into a global conflict, and the possibility of a Jewish worldwide spy ring intent on taking over the world. It's stuff that's a creative exercise at most. No worse than if I asked you what you thought about the moon landing being real or fake. I don't see people getting righteously pissed about others saying that was fake. Yes, I'm comparing that to this because it's the same thing at the base level.