The privatization of prisons—whose business model depends on locking up more and more people, since penalization is their "product"—has an opponent that isn't usually known for its liberal prison-reform work: prison officers.
Or more specifically: prison-officer unions, which stand to lose jobs and influence if government begins to cede an entire wing of its responsibilities to private business. And let's not forget that private business doesn't just have a prison-management buck to make off government contracts—it has a whole potential labor force of incarcerated people, who are already working below minimum wage for no benefits and "siphoning away" big government contracts, such as sewing military uniforms. (The current contracts go to a government-owned corporation called Federal Prison Industries/UNICOR, founded in 1934, and other corporations want in on the action.)
Britain has been dealing with similar problems—prison privatization, restive prison guards—so is training its military to fill in as jailers in case of strikes:
Hundreds of British army, navy and air force personnel are being training as backup prison guards amid fears of a strike at some correctional facilities...
Prison officers are forbidden by law from striking, but they have conducted wildcat strikes. Prison officers at an estimated 80 percent of facilities in England, Scotland and Wales participated in a surprise walkout in May.
This situation will only get stickier as it goes along—a federal government business-monopoly vs. private business, state governments in negotiations with private business, government-employee unions vs. private business, the military training as union scabs... A graph of these intertwining interests and rivalries would look like a plate of spaghetti.
The one big constituency absent from the conversation? The inmates/prison laborers. And why is that? Can't they organize and form prisoner-labor unions to represent their interests?
Of course not! While that isn't explicitly illegal (as far as I've found), the legal barriers are, in the words of Prison Law Blog, "basically unsurmountable." If you're curious about that particular plate of spaghetti, they explain more about it here.
At any rate: Prison privatization is a mess that's going to get messier.