Good to know that the Chinese exchanges don't have a monopoly on Bitcoin trading.
Japan may be number one now in volume but Chinese exchanges enjoyed a highest volume for a long time and i know in future they will retain the highest volume again. Seeing other countries like Japan in the Race with china in Bitcoin adoption is sure a healthy competition for bitcoins overall.
Those high volume was fake. You can see how fake it was when those big exchanges implemented fees on trade transaction. But yeah, I agree that world wide adoption and competition is healthy for Bitcoin economy. As far as I know Japan is more Bitcoin friendly than China.
The popularity of bitcoin in Japan started evolving last year as there was a growing demand for bitcoin and things might even change in Japan in the future as there are rumors that Japan is planning to legalize bitcoin as a currency. There may be new rules and regulation coming up and really looking forward to how the future holds.