D etective Sergeant Steve Riback of the Las Vegas Metropolitan Police Department calls himself the “Kosher Cop” and it’s not just due to his dietary restrictions or to the years-long court battle he waged against the department to accommodate his First Amendment religious rights to wear a beard and yarmulke on duty. In “Sin City” — the cash and casino capital of the world — being a high-profile Orthodox Jewish cop means giving a moral compass to this city of high-stakes gambling and all the vices that go with it.