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…
They're insanely rigid, almost always put arrested people in detention for maximum of 23 days even if they know the people are innocent, and they have a 99% conviction rate.
After living here for a couple of years, I feel that this is also closely linked to the highly homogeneous culture, where people are generally very friendly and kind to those who share the same origin, and can become unconsciously biased against people or things out of the "normal" world.