You make a good, and troubling, point. I would merely point out that to date, the Al Qaeda cells and wanna-be's have either been Black Muslims (with whom mainstream Islam has little to do with), random doofuses (John Walker, Mr. Shoebomber, etc.) with whom mainstream Islam has no relationship, or immigrants (who exist in their own subculture.)
That said, I concur that the deafening silence coming from the mainstream Islamic community in this country is an indictment against them, and I have previously posted that to be silent means they (the mainstream Islamic community) are condoning the Islamofacists. And if they don't start speaking out, they will be condemned for their silence, for their silence will be interpreted as affirmation. And if and when that happens, upon their own heads be it.