They're not prisoners until they're captured and put in prison. Thus lobbying for more prisoners is a way of exploiting more people, especially the poor, young

They're not prisoners until they're captured and put in prison. Thus lobbying for more prisoners is a way of exploiting more people, especially the poor, young black men who are most likely to end up in prison. Similarly, lobbying for the capture of more slaves on the west coast of 18th century Africa is a way of exploiting Africans. Additionally--and I suspect this is what the GP was going for--the lobbying effort clarifies the underlying intentions. The people demanding that prisoners work are doing so not under the banner of exploitation, but under rhetoric about prisoners "giving back to society". That prison organizations are lobbying for more prisoners reveals their real intentions, which is why it's used as evidence of the exploitative nature of the penal system.