Well I guess you better go tell that to Dearbornistan.
Where there are large mussie enclaves they practice sharia. Just like some Orthodox Jews use their own courts. That is not yet against the law. If said people agree to have a separate panel pass judgment on their disputes there is ZERO you can do about it
Most Jews NEVER use "Jewish courts" ( which aren't really "courts" at all and usually ONLY deal with divorce cases ) and even the Hasidim ( who are the ONLY Jews who now use Rabbical decrees, vis-a-vis divorces ) also ALWAYS get legal, American court system divorces! The only reason one needs a "git" ( and it's ONLY the woman; IIRC ), is so that they get the child support promised them by our legal system, have custody of the children, and/or when they want to remarry.
You really can't compare the two things at al,l.