I am Norwegian, so I thought I'd add a few points, which I think greatly influence the difference.
1) Prison guard education is just a few weeks in the US to my knowledge. In Norway it is a 3 year college education, where you learn about mental illness, why people become criminals, how to rehabilitate people etc. The prison guard job in Norway, is by American standards more like a social worker. Guards spend a lot of time doing activities with inmates. They are not simply there hit misbehaving inmates.
2) The criminal justice system is largely under professional control. We don't elect sheriffs, judges etc like in the US. So there is less opportunity for populist "though on crime" politics to win through. Policies are mainly driven by professionals in the field.
3) Norwegian culture isn't as hung up on good and evil as American culture. I guess that is partly because religion has little importance in Norway. Christianity is quite focused on the idea of good and evil. Norse mythology was never like that. There was no evil devil. The world was divided into order and chaos not good and evil. The world "evil" almost never surface in any discussion. It isn't just the justice system, but Norwegians themselves aren't very interested in revenge. E.g. I've never heard a single survivor from the Utøya massacre express a desire for revenge.
I don't know why it is like that, but perhaps a reason is that Viking culture was very revenge and honor based. But this mentality was destroying society so authorities worked hard for hundreds of years to change this mentality.