Hello, the problem is not only their religion, it's everything that led them to adopt islam these last months/years. Check their bios; they are immigrants who fled war in their homelands. Their parent moved and brought them in north america wanting a better life for them; they studied, worked, but never really fitted in the western, white world. Over the years they developed a disgust for the western way of life, even if they probably loved growing bathing into it during their childhood and teenage years. At some point, after some traumatic events happened in their lives -death of a friend as reported, maybe a gf dumping him or something along those lines- it triggered a complete rebutal of the occidental way of life, and instead of killing himself dude discovered a coran put there by someone else - a muslim relative?- started to browse the internet for islamist stuff, videos etc, and he decided to give a new direction to his life ; now he had to purify the world, the earth, in the name of allah, and he was given the order to kills white people, the same white people who never gave him the time of the day because he was a little tan and arabic/muslim looking...
#1 I wouldn't have let them come to USA in the first place. Yes, you're in a warzone over there, that sucks, but we don't have the $ and security to accept you.
#2 Maybe once you immigrate here, you agree not to travel back and forth? Weird and tough, but limits them going back home to train, etc.
#3 Maybe IF you travel back and forth, you're monitored like crazy.
Beyond that, I think the FBI made a HUGE mistake by having such a weak LEO presence in Boston to begin with - and by not releasing the identities 24 hours earlier. This whole "We don't want to tip them off" gave them 24 more hours to sleep, build more bombs, etc.
I'm not defending their beliefs in any manner - I'm just saying focusing upon that doens't help... Keeping them them $*^&^%*$ out of the USA in the first place does.