KWR is correct. JWs, and all other religious groups that hold services in prisons, are protected under the Religious Freedom Restoration Act (RFRA) and the Religious Land Use and Institutionalized Persons Act (RLUIPA). These acts state that prisoners must be accomodated so that they can practice their faith despite being incarcerated.
The Des Moines case differs in that state funds were being used, in essence, to promote one religion over another, thereby establishing a state religion, which violate the Establishment Clause.