Stepping aside the fact that I think most everyone here is playing fast-and-loose with the “slave” terminology here… Why do you feel prisoners doing low wage labor to be wrong?
Practically everyone in human history since the dawn of time has had to go out and produce something of value. Why, all of a sudden, should a murderer or rapist get to sit on their ass and consume what we all produce? I find nothing questionable about a humble job for them at all.
Working should be a free choice (we can discuss about how much freedom exists for many people), and should always be paid. There is nothing wrong if a prisoner chooses to enagage in (fairly paid) labor. But if they are not free to do so, then they are slaves, not workers.
Prisoners already lack freedom in many aspects. "Sitting on their asses" like if they were sipping cocktails on a beach is a bit a misrepresentation don't you think? I wouldn't exchange the possibility to move and do what I want for possibly any amount of money, nor for being able to "sit on my ass" in that sense. Would you?
Besides the moral arguments - which I will say, they are so obvious that it feels incredible even having to discuss why enslaving prisoners is wrong - you can make economic arguments. For example, that having cheap or borderline unpaid labor compresses the salary in that market, or that this system creates a dysfunctional incentive to increase prison population for private profits.
Maybe that's why the US is one of the countries with the highest incarcerated population in the world. The highest among western and larger countries.
I understand though there is a cultural barrier. I am from Europe and in most countries here prison has a rehabilitation purpose, which is what most benefits society, and prisons are not private entities.
I don't think you know what humble job means, and meant to say humiliating pay.
Two answers:
1. Why should they be restricted to ludicrously low wages? If they're producing something of value, they should be compensated. Not only is it morally wrong to, you know, enslave people, on a more practical level it would be very helpful for people who are leaving prison after serving their sentence to actually have some money saved up, so they have better opportunities, to avoid recidivism.
2. The reason they can sit on their ass and consume what they produce is that they effectively become wards of the state. They're still human beings, and if we have decided to incarcerate them, we become responsible for them, and they still have rights as human beings.
A humble job is fine; I'm not saying they should be sitting in an aeron chair bullshitting on Slack for 8 hours a day. But slavery for pennies on the hour is wrong.
You're instantly jumping to the worst of the worst types of prisoners: murderers and rapists. Prisons also include people who commit non-violent crimes like drug possession, burglary, cybercrime, etc. Why should those people be forced to work the same "humble jobs" in prison?
I do find that questionable.