> That said, prison labor is quite common across the globe and considered rehabilitating.
I cannot speak for Finland specifically, but in many places prisoners are taught and can earn qualifications while in prison. If they can leave prison with qualifications as a cook, car mechanic, car painter, carpenter, electrician, or even locksmith (yes, I've seen that, as ridiculous as it sounds), that might actually help them.
However in the US prisoners are often exploited to perform utterly menial tasks. Forget any formal training or earning qualifications. Even if they would perform the same task outside prison, they probably couldn't compete with $0.10/h prison labor. If that cheap human labor didn't exist, those tasks would likely just be automated.
So there's a bit of nuance there and I wouldn't consider all kinds of prison labor rehabilitating.
The philosophy in the US seems to be that you just have to change people: Instead of giving people opportunities to lead a normal life after prison, options are actually taken away from felons. Most other western countries recognize that many crimes are also a result of circumstances and lack of opportunities. The easiest way to get people to follow your rules is not punishment, it's making them want to do it because conformity is the path of least resistance.