By not assimilating into society they maintain their hatred for the West.  The bomber son was a product of the parents and the Muslim world.  Allowing immigrants from countries hostile to the Western world is inviting this kind of trouble.

Muslims all over the world, not just the U.S., do not assimilate into the "mainstream" culture.  They live and work in their own Muslim communities.  Being born in those communities is the same as being born in Libya or other Muslim dominated countries.