I don't get how the faith of a group of people can be insulting to anyone. Of course it was islamic extremists that were responsible for the attacks. But extremists exist in every religion, they don't speak for every member of that community... they're not even the majority. They're just the loudest ones.
What is insulting though is the fact that there actually were a good amount of muslims among the victims of 9/11, yet members of that faith are treated like the worst perpetrators. They were victims too and they have the same right to build a community center near ground zero like any other religion. I remember right after the attacks, it didn't matter where you came from, what you did for a living... what faith you had. If you were in NYC at that time or lost loved ones in the attacks you just belonged there you were part of it. "We will never forget", apparently some people already have forgotten.