You seem to have remarkable insight into the things unions "care" about. If they only care about increasing membership (and dues) why don't the unions simply come into the private prisons and organize the guards there?
OR- maybe you're completely wrong and you have no clue. David Meyers (CCA's President, West Coast Region) repeatedly raises this same issue. He has regularly suggested that the union in CA stop fighting CCA's plans for CA and CCA will allow the union to organize the guards.
If it was only about union dues, the unions would probably organize the private guards, and they would not need CCA's permission to do so. Rather, it is about itrying to improve the profession, improve training and standards, improve work conditions and improve career incentives.
Some unions have allowed PAY CUTS to go through in order to fund increased training. Others have pursued legislation to mandate pre-employment psychological screening. This does not sound like dues above all else either.