I recently heard a speaker who shed a lot of light on the Islamic faith. He said that they believe that no matter how wonderful a life you live, you never have assurance of "heaven". It is up to the sole discretion of Allah as to who gets in and who doesn't. While leading a good life is certainly encouraged, it doesn't assure you of paradise.
One of the few ways an extremist believes he can have certainty of paradise is to have killed an infidel, one who is against Allah.
Possibly those men knew, because their planned deed, that they were going to be good to go, no matter their pre 9/11 activities??