Weren't we comparing the rate of incarceration of the Japanese in Japan vs the US?

Japanese Americans are a group that has a sizable population of people who have lived their entire lives in the US. If you think that they would be more likely to behave similarly to people who have spent their whole lives in Japan, the burden of proof is on you, because that certainly is not a mainstream view.

1) Confusion between Japanese in America and Japanese-Americans

2) "More likely to behave similarly" is a considerably wider net than just crime rates

3) I think the mainstream view is that East Asians of all stripes commit considerably less crime than average. It's certainly true of violent crime, which is something I've looked in to.

These are Americans, not Japanese.

That is an interesting way to slice it.

The implication of pointing out the US' high incarceration rate is that the US justice system and society are particularly malevolent.

This point was used to defend Japan's justice system as being far more benign than the US.

However, Japan's justice system may in fact be far more unforgiving that the US' if Americans are, in general, more likely to commit crime. Which is perfectly fair point to make (although inconclusive for various reasons).

If (for whatever reason) Americans are more likely to commit more crimes, they are, all else being equal, more likely to be in jail.

Of course many things can be true at the same time. Japan can have a brutal justice system and have a very docile, law abiding population. Maybe the brutality caused them to be docile. Or, America can have an abusive justice system, and a population that needs to be more incarcerated than others.

Either way, the criminality of the Japanese is a relevant factor if America's incarceration rate is made relevant

Yeah, but the parent comment is talking about Japanese Americans, not Japanese in America. The experience of Japanese Americans is not related to Japanese people (especially if raised in Japan).