I guess you mean the ulterior motive! The government does nothing ever for free motive. I agree to that. But in Japan's case, I think, it's because of the Greenback and his close ties to the price of the Yen; with cutting taxes on BTC and the side effect that many citizens are selling Yen for BTC the US $ has less influence on their economy; since WW2 Japan is nothing but a vassal state to America!