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.