This is an important point that not that many prisoners are in for-profit prisons, but many of these groups are also lobbying to privatize existing public priso

This is an important point that not that many prisoners are in for-profit prisons, but many of these groups are also lobbying to privatize existing public prisons so they're not an inconsequential influence. They also benefit at the margins a lot. (i.e. if one extra person is incarcerated they're likely to go to a private prisons since the construction of public prisons has slowed a lot).