I had a vision of the experimenters being prosecuted for wasting police/people's time by causing problems on purpose.
Edit: the above was a joke, but had a grain of truth. The authors addressed this specific question in a reply to readers' letters:
We did not exclude Japan from our study because their reporting rate was too low (if anything, Japan would have scored relatively high among countries in civic honesty). Since our focus was on investigating the difference in reporting rates between wallets with and without money, our decision rule was to exclude countries where individual wallets could not be identified. Japan has a unique system with lots of small police booths (Koban system) where citizens return lost objects. While we found a large number of our wallets listed on official lost and found websites, we were not able to retrieve these wallets as it would have required proof of ownership and identification documents. As a result, we were not able to find out the unique email addresses or the original locations of the wallets and we could not ascertain (among other things) to which experimental condition the wallet belonged. As we were primarily interested in the difference between experimental conditions, we had to exclude Japan from the paper. By contrast, we do not have evidence of similar systematic behavior from other countries.
Actually one of our research assistants was temporarily detained (in Kenya, I believe) for suspicious activity. As you can imagine, having a bunch of "lost" wallets on hand required some explaining.
Edit: the above was a joke, but had a grain of truth. The authors addressed this specific question in a reply to readers' letters:
We did not exclude Japan from our study because their reporting rate was too low (if anything, Japan would have scored relatively high among countries in civic honesty). Since our focus was on investigating the difference in reporting rates between wallets with and without money, our decision rule was to exclude countries where individual wallets could not be identified. Japan has a unique system with lots of small police booths (Koban system) where citizens return lost objects. While we found a large number of our wallets listed on official lost and found websites, we were not able to retrieve these wallets as it would have required proof of ownership and identification documents. As a result, we were not able to find out the unique email addresses or the original locations of the wallets and we could not ascertain (among other things) to which experimental condition the wallet belonged. As we were primarily interested in the difference between experimental conditions, we had to exclude Japan from the paper. By contrast, we do not have evidence of similar systematic behavior from other countries.