Religious law courts exist in this country already. JWs, Mormons, Catholics, Jews, Presbyterians---to some extent most religions have their religious requirements and some kind of enforcement structure.
The kind of sharia court you are all afraid of-like the village elders in small corners of India or the backwoods of timbuktoo are never going to have a legal foot in our country, but religious courts that decide on whether a person can be RELIGIOUSLY married or divorced are not going to be respected in our law when they attempt to supersede secular law. For example, a JW will always be able to marry whoever they want. A Mormon, an Atheist, a Venusian. Whether they will be acceptable to the JW community is another thing altogether. A JW child can be DF'd, but LEGALLY (unless parents play games), the parents cannot kick a minor out of the house and say they are within their religious rights.
Many fundamentalist Christians these days are marrying without secular or official licenses. They are living together (or become common law spouses eventually in some states), but are not considered by their peers to be "living in sin" since they pledged their lives to each other in a public way or announced it. Religious law will always have rights in thsi country so long as it doesn't infringe on secular law.
A "gang" of any group/religion/ethnicity/sports club that is not controlled by the local government is just a gang that is operating outside of the law. If the local authorities let it go on, shame on them.