The iPhone is really big in Japan (it's one of the few countries where it has a higher market share than in the US) which probably makes it more worthwhile for Apple
Since it's not free for them, I assume they determined the amount of people in their key demographic that visit Japan was worth providing a stellar experience. Considering 2.7 million Americans visited last year, it was probably a bet that's paying off.
¹ Or, incidentally, people who live there but purchase phones overseas to avoid an obnoxious camera shutter sound forced on at all times.
I'm guessing it's largely the working class and undocumented immigrants. And probably lots of burner phones?