As someone who lives in Japan I believe this is not a result of "Japanese people being more honest", but rather: - Availability. There are police stations (kobans) and officers on the street everywhere, just a few steps away from wherever I may find something. Turning in a lost item has no friction. All it takes is a few extra steps. In the U.S. there is huge friction - I would need to go out of my way to go to a pol…
Japan has a 99% conviction rate and indefinite uncharged detention happens often. Despite that people treat police amicably, and there is a trusting relationship. That's to me a sign that there is a fundamental cultural difference in Japanese attitude, in the US stats aren't anywhere as close to as severe, yet relations/trust is much worse. Goes to show how much of a productivity and effectiveness loss lack of trust…
And while the kangaroo court thing is maybe a bit too true to be comfortable with, I'd wager Japan errs heavily on the side of not bringing potentially contentious cases to court.