From what I’ve been reading they have a lower incarceration rate than most countries and their penal system is focused on rehabilitation rather than punishment. Who knows how true that is without personally visiting one, but the US prison system is known for being awful and focused on punishment, with some of the highest (I think currently the highest) incarceration rates.

I don’t think most prisons around the world are “good”, likely far from the rehabilitative facilities most leftists would like, but the US penal system is likely worse than Japan, at least in regards to the prisons.