It's sillier still to suggest "they" are saying they do know for sure, when all "they" are saying is- very slowly for the kids in the short bus At Presssssent Therrrre issss noooo Evvvvvvidence for terrorism- i.e. We do not hear Achmed breaking into the cockpit screaming "death to American and Michael Jackson" We do not have reports of a taliban with DsHK twin mount in the back of a toyota firing into the sky, we do not have reports of another aircraft ramming the plane, etc.
No evidence at present doesn't mean evidence won't be found later- all it means is no evidence right now.
Well, there is the fact that "Planes don't crach into Queens!" as one poster put it- perhaps that is all the proof we will ever need...