If it was just because of the genre, their brother would notice that anecdote everywhere and not just Japan. The drummer in my current band plays with several groups (very common for a skilled drummer in NYC), and he just got back from a 2 week tour in Japan and had the exact same things to say. Crowds were way more respectful and focused on the music.

I agree though that in general niche music is more respectful than more mainstream music, at least in the USA. Harsh noise and stoner sludge/doom shows have had the most polite people in the mosh pits in my experience.