As someone with similar experience, I disagree completely. It lies deeply in Japanese culture and is thaught since early days. Proiximity to the police station has nothing to do with it.

If anything, they will leave things on the street if there is no koban near by. Every single kid knows that, if you see some unattended property, the safest option is to leave it be. If it's a commodity then you can return it and probably should. Everything else you don't touch (and by any circumstances you don't touch bicycles!)

I don't remember the thread, but there was a story here on HN about the guy who tried to take and return the bike to the police station. He was, to his own surprise, questioned hardly about why did he do that and what was he thinking. They wanted to put him in detention, too.

I do agree about police helpers, though. I am not living in Japan anymore and, boy, I am missing that.