What do the statistics mean?
Are Norwegian prisons more effective than United States prisons? These statistics do not exist in a vacuum. Prisons have very little to do with rates of first offense. To the extent that deterrence matters, it is far more a result of law enforcement. In particular, the chance of punishment has a greater deterring effect than the strength of the punishment.
The US has a much higher offending rate than Norway. Consider the possibility that the recidivism factor is the same for both countries. But the higher offending rate produces a higher recidivism rate.
What's a recidivism factor? The amount that being imprisoned makes someone more or less likely to offend. It's the thing that prison can most affect.
A lower offending rate makes all the other rates look better. It reduces the incarceration rate directly. A lower offending rate also means that there is less pressure to protect people from the criminals by incarcerating them longer. Which also reduces the incarceration rate.
A lower offending rate reduces the recidivism rate as well. Someone who is less likely to offend the first time is also less likely to offend a second (or later) time. And of course, someone who never offends a second time won't offend a third or later time.
Population differences
In Norway, about a third (34%) of all crimes are committed by foreigners. Example source. These are people who can be deported after their sentence, giving them a very low recidivism rate.
In the US, immigrants are less likely to offend than the native born. Even though it is much easier for deported criminals to get back into the US. They just need to cross one land border. It's much harder to get through the controls for air and sea travel in Norway. This is especially true since most of the deported criminals aren't from adjacent countries but from places that are far away.
This suggests that Norwegians are simply less criminal than the world average. Interestingly, the US state with the largest Scandinavian population in total and second in percentage is low-crime Minnesota.
More statistics
Average sentence in Norway is eight months.
Average federal sentence in the US is thirty-seven and a half months. Some other sources say sixty-three months. The difference may be between the sentence at trial and the time actually served. I.e. they may be sentenced to sixty-three months at trial and only serve thirty-seven and a half. Or the difference might be federal versus state.
Either way, sentences in the US are four to eight times as long as in Norway on average. Some of this may be that the crimes are more serious (if Norway were a US state, its murder rate would be in the bottom ten), but a lot of it seems to be that sentences are longer for equivalent crimes.
The natural experiment
If it were true that recidivism were primarily a result of prison policy, we would expect that the states that were politically closest to the Norwegian policy would have the lowest recidivism rates. Those states would be the ones run by Democrats.
Three-year reincarceration rates by state (PDF).
The lowest rate is in Virginia, a traditionally moderate state that is just starting to lean liberal. Traditionally Virginia has had moderate politicians of either party. So law and order Democrats.
After that, we have Oklahoma and South Carolina, both law and order states. Then the more liberal Minnesota, with its large Scandinavian population. Florida, West Virginia, and Ohio are not known for their liberal policies. Oregon perhaps.
If we skip to the bottom, we should see a bunch of Republican states, right? After all, most states are Republican leaning. But Delaware is not a Republican state. Utah and Alaska are, but Connecticut, Rhode Island, and Vermont are not.
Both California and New York are in the bottom half but towards the middle.
These are not results consistent with the claim that liberals are better at rehabilitation than conservatives. In fact, it is more consistent with the exact opposite conclusion.
Summary
We don't know.
We don't know if the Norwegian prison system is better. Although we do know that their results are better. We don't know if that is because of the differences or in spite of them.
We don't know why Norwegians are less likely to be first time offenders. We don't know why the few that are imprisoned are less likely to reoffend.
All we do know is that within the United States, the states with the three best recidivism results are nothing like Norway.
Anyone who tells you that they understand why the US is different from other countries on crime is guessing. They don't actually know because no one does. Their guess may be correct, but it probably isn't. There are a lot of mutually exclusive guesses. Only some of them can be right.
The US has a much more serious crime problem than does Europe. Why? Unknown. People comparing US policy to that of individual countries in Europe have to explain not just the difference between that country and the US, but also why the same comparison does not work when you swap the US with other countries with the same policy. And further, they should explain why US states are not able to get similar results, even though prison policy differs greatly between states.
bad guys more inmates kept longer and more likely to go back. That kind of comment assumes that criminals are criminals because they are "bad guys" and will keep being criminals (because "bad guys"). Of course, the logical conclusion is that any crime should be punished with execution (after all, it is cheaper and they are, well, "bad guys" who will keep committing crimes). But Norway statistics tell a different story...