It's a "frog in boiling water" situation, it's gotten worse so slowly that there's no one incident to cause a mass outrage. But things like the letter in the original post might come close.

https://upload.wikimedia.org/wikipedia/commons/thumb/5/54/US_incarceration_timeline-clean.svg/1200px-US_incarceration_timeline-clean.svg.png

You can see how much the "Drug War" from the 1980s and Reagan caused the prison population to explode.

Another part is that private prisons are only about 5%, yet there are many public prisons that are basically for-profit in practice, but they're still called "public" prisons. No one really talks about those. That's where they're basically working them as slaves to produce products they can sell, like furniture. All furniture in public schools in iowa was made by prisoners, I've heard. They even have a website where you can shop products! https://www.unicor.gov/index.aspx

Furthermore the public prisons get paid by the gov't on a per-prisoner basis, meaning they have a huge incentive to cram as many prisoners as they can in to their prison, without building more infrastructure. So almost every prison is over-crowded.

This situation is an absolute joke. The US is currently setting the world record for number of prisoners, both in raw numbers and per-capita.

I don't know how anyone can objectively look at this and see it as acceptable.