How about a Satanic sect that believes in child sacrifice, the enslavement of women, and the murder of all Christians and Jews...
Would you defend that with your life?
Example: It is currently illegal in Texas to have more than one wife. I however, if I so choose am free to believe that I deserve 6 wives if I'm feeling masochistic at the moment. I don't ever forsee a situation where it will be legal for me to have 6 wives, but the state of Texas cannot, and more importantly should not even try interfere with my underlying thoughts and motivations. The state by necessity has to stick to regulating the action, not the idea.
Another well known example would be the white power nutbars to believe that the "mud-races" should be exterminated. Now, as a short, brown eyed, dark skinned man, that troubles me on a personal level, but unless they're acting on the belief, what possible recourse could their be?
Now to bring this back around to religion. A brief lesson history, would be the creation of the maranaros in inquisitional Spain. Spanish for swine, the maranaros were Jews who adopted the outward trappings of christianity in order to keep their physical personages intact. Did Spain succeed in converting these Jews? Of course not. Even the most iron handed of despots cannot change the heart or mind of a single man. All it succeeded in doing was driving it underground.
If anything, I would fear an underground and oppressed Islam in America more than I fear the open observation of Islam within our borders. The former would be much, much, much more dangerous; driving many who would otherwise be peaceful to new heights of radicalism.