Because Muslims have a codified doctrine on the use of violence for the purpose of extending their influence and Christians and Jews do not. Your question is as self-refuting as asking why we should legitimate businessmen and racketeers be held to different standards.
If islam were not a religion, what would you call those who practice it?
There are certain extremist groups with Christianity and Judaism that have codified their endorsement of violence to achieve their theologically driven objectives. At this particular point in history, those are quite small subsets of those larger religious traditions, but Christianity in particular once had a widespread devotion to violence as a means of extending its influence. Islam can change too, but we can help or hinder the pace of that change, by our choices in how we treat moderate and liberal Muslims -- the more we alienate the moderates and liberals, the more susceptible they become to the welcoming advances of the extremists. Personally, I don't want to help the extremists in any way.
Until we know more about this particular family and young man, I think it's important to keep in mind that "Muslim" is just the default religious identity of the entire Bosnian ethnic group, and the fact that a Bosnian identifies him/herself as "Muslim" when asked what his/her religious affiliation is, doesn't imply any serious level of belief in the teachings of the Bosnian brand of Islam (which is a notably non-extremism-prone brand to begin with). It's sort of like asking a native Englishman what religion he is -- if he doesn't give a whit one way or the other about religion, and hasn't been to church in years except to attend a few weddings and funerals of friends and relatives, he'll generally say "Anglican" -- but trying to make a connection between Anglican theology and any particular acts of this individual (good or bad) would be groundless.