That prisons are or should be about justice in the first place seems worthy of skepticism. It seems to me prisons have many possible uses. Rehabilitation is a popular one, as is justice/retribution/revenge/punishment/deterrent (these all seem like different perspectives on the same concept.) These two broad motivations for prisons, rehabilitation or retribution, often dominate discussions about prisons, but what of the simple utility in keeping dangerous people away from those they might want to harm? A prison need not be inherently punishing to achieve this end, nor does it necessarily need to be rehabilitating (though rehabilitation would help keep costs low in the long run.)