From the beginning of the article it looked like the complaint was that someone was trying to get a job with the prison to promote whatever religion and was denied. I'm sure they don't have it in their budget to hire people to represent another 5 religions that maybe 20 people out of the entire prison population would be interested in. If this is the case, I see no wrong doing.
Besides, the constitution says you have a right to religion, but says nothing about the government providing you a way to practice your religion. What they are already doing in the prisons is a courtesy that they don't have to be doing in the first place.