(This is a rhetorical question. It's not fair to blame clearly unhinged extremists on a religion at-large when billions of devotees do no harm.)
Because that doesn't actually exist and it's just a faux and very poor argument.
If you are intellectually honest you would understand there is a big diference between someone from a certain group (may it be christian, muslim, etc) just committing a crime as an individual and someone from a certain group committing a crime invoking principles that derive from belonging to that group (like their religious supremacy for instance).
In that regard, yes, there are a lot more islamic attacks than there are christian attacks. We had one single attack in Europe that could (emphasis on could since the guy was by foremost an extreme right winger and not some christian fanatic) be connected to christian ideologies in the past decades (by Anders Behring Breivik, which did in fact kill a lot of innocent people)... but now go and check how many terrorist attacks you have connected to islamic ideologies.
Besides, religious courts already exist in the US. For example, Halakha are religious courts run for Orthodox Jews, they exist in America and make many decisions on family law and civil disputes. But even among bigger groups, lots of disputes are dealt with within the religious community, Catholics have 200 diocesan tribunals, if you want to end your marriage and stay in communion, you pretty much have to ask for an annulment at a diocesan tribunal.
Lots more examples:
http://www.pewforum.org/2013/04/08/applying-gods-law-religio...