Legally speaking seems like in the US that prisoners are slaves.
SOURCE: https://www.loc.gov/rr/program/bib/ourdocs/13thamendment.htm...
https://thepiratebay.org/torrent/16102495/13th.2016.NF.WEBRi...
Much of this theory is based on The New Jim Crow: Mass Incarceration in the Age of Colorblindness by Michelle Alexander which first popularized the idea of how we utilized the prison system to intentionally discriminate and keep locked up (for free labor) the african-american populations.
There recently was a book discussing a different explanation. In this book, Locked In: The True Causes of Mass Incarceration and How to Achieve Real Reform by John Pfaff, Pfaff argues it's the perverse incentives of prosecutors and judges that are pushing up the prison population.
I don't know enough to say which is true, just wanted to get awareness of the competing ideas on the topic.
But definitely there's some really perverse incentives to lock people up for free labor. And, as the documentary accurately points out, it's all built in to the Constitution.
Nor does the existence of perverse incentives mean that those presented with them are helpless to do otherwise than go along with them. That is an abdication of moral responsibility. If you are presented with a perverse incentive that you feel is incompatible with your personal moral calculus, then it is your duty to reject it. To go along with it and then say you had no other choice is no different from the behavior of the 'good Germans' described by Hannah Arendt in The Banality of Evil. It's literally the exercise of privilege.
Liberalism only works so long as people are invested in the integrity of the institutions which underpin it. When those institutions are corrupted, liberalism becomes complicit in oppression. This is how democracies devolve into autocracies.
That's why the Hollywood version sometimes has the judge say, "I sentence you to ten years hard labor at Camp Rockcrusher." If they just said "ten years confinement in Oubliette Prison," then the prisoner would not be compelled to do anything against their will.
Instead, we oversentence criminals, and use the low-wage prison jobs as a carrot to reduce their prison time. You can work for peanuts and get out early, or you can serve your entire sentence.
The net effect is the same. The rent-seeking profit motive makes the criminal justice system more cruel and exploitative.
Also, the word "except" is our key hint here.
I seriously doubt (and see no evidence for the idea) that the US is so lawless that we can have almost a quarter of all prisoners in the entire world while only having a fraction of its population.
This speaks strongly to the idea that huge amounts of wrongful imprisonment occurs and (as a logical consequence) these people are slaves against their will without their having done anything worthy of punishment.
The difference is solely the standing law. During the nazi regimen (godwin's law, but really) many of the appalling things they did were legal. It was legal to execute, and it was legal to do forced labor.
Slavery was always legal, that was the problem all along! A slave could not physically escape without the fear of unpunished retribution.
Redefining the word so that prison slavery doesn't qualify seems obtuse, counterproductive, and doesn't even have the defense of being historical.
If locking people up and forcing them to work is making people billions, it's slavery. Period.