The companies and people that run the prisons and the prison phone systems are both very happy with a system where a) people spend a lot of time in jail and go back frequently and b) spend a lot of money on hyper-expensive phone calls since both of those things earn them very significant money.

The question should probably be: Why should any sane person in the prison industry be for it?

They get commissions on the calls, which often directly benefit the employees.