I'd fathom the theory, personally, that these certain experienced pilots either encountered some external influence that makes them disagree (a simulator limitation, for instance, that does not allow a simulated flight beyond a certain airspeed without losing control), have a personal gain, have a disposition for conspiracies in general, or simply lack knowledge in certain areas. That's just my opinion though, so agree/disagree as you will.
I don't know these pilots, but I've met pilots of all types throughout the years. If you ever read crash investigations, or even watch Mayday on television, you'd see even experienced pilots making critical rookie mistakes or being wrong about something important. And when it comes down to it, experienced or not, these pilots are still just people with different personalities and knowledge. I've met ATP level line pilots of who are great pilots I am sure, but struggle with certain basics like understanding the lift formula. I've also met pilots who don't quite understand their aircraft as well as they should. So, opinions can come from all types, experienced or not.
With regards to 9/11, at the end of the day, you have to look at the evidence available and ask if it makes sense. Just using newtons law, and the fact that by the time a student pilot takes his 5th or so lesson and should be able to line up with a runway, it just seems perfectly reasonable to me that indeed terrorists could have planted a plane into a 207 feet wide skyscraper. Yes, they were flying beyond safety limitations, yes they weren't type-checked pilots, and yes, it'd be nice to see those black boxes in NY, but in the end, those things don't seem as relevant in comparison given other alternatives and what people saw.
Just my opinion.
