In the US, that is constitutional, though still a terrible thing. https://constitution.congress.gov/constitution/amendment-13

That’s true, however the nearest analogue to non-criminal forced-labour ‘prisoners’ would be the military - and in the UK that makes deductions for food, housing and utilities.

Top of my head, I’d estimate an effective hourly rate of about £1 to £2 for a week’s work at minimum wage minus payment to the prison for accommodation and food - but then there is an issue of whether prison labour is of real economic value and not often a form of ‘make work’ ie is sewing mail-bags really worth £10 an hour? Perhaps 50p per hour (net) is actually a fair return for value produced?

If work doesn't provide value that is higher than minimum wage it is work not worth doing. It is not like we must provide prisoners work. We could offer them free education too instead. And not even pay them for taking it.

It might be worth doing if it’s of psychological or behavioural benefit to the prisoners ie it prevents boredom / fighting / depression, and inculcates an ‘employee state of mind’ (usefulness, discipline, and ‘earn to spend’).

That’s fine if the state was providing these activities as unpaid or low paid hobbies in addition to actual jobs.

The problem is that the state is exploiting the prisoners for profit. The profit should go to the prisoners doing the work.

But isn't prison special and exempt from our distaste against slavery? How could it ever not be akin to slavery even if they were paid?

According to the article there are not enough jobs to go around, so you are not forced to do a job.

However, if you do take a job, you should be paid for it, like everybody else.

This is important to counter prisons for profit schemes as well as improving the wellbeing of the prisoners.