For profit prisons are unethical.

Private prisons need customers, customers, meaning inmates. They only make money by locking people up, and the more people they lock up for more time, the more money they make.

Private prisons are morally repugnant, don’t save money and have performance problems. But they persist. Why does this practice continue to grow?

The answer is not the lobbying or the political contributions — though the industry does both. The answer isn’t the contracts that guarantee 90 percent or 100 percent occupancy — though surely they help — or even the industry-financed research that purports to show cost savings when neutral research shows no such thing.

The answer is deeper than that and goes to the very heart of the industry.

Private prisons are filled because they are there. They succeed by being available. Companies build them, and people come.