How did this become the #1 free iphone game in Japan?

A lot of japanese downloads.

I know in games like Starcraft players are obsessed with their Actions Per Minute (APM); the more you click the higher your APM and the better you do in the game.

Yes, you can see a whole host of problems with this logic.

None the less, my impression is that it looks like this game hooks into that sentiment and tests your ability to react quickly.