The cool music you can't buy on iTunes, unless you're in Japan
This isn't unique to the Japanese iTunes Store, of course, every country specific store also has a bunch of songs you can't buy outside of that country's iTunes Store. Collins wonders why these borders are enforced for digital downloads, but I imagine it is another sign of the Recording industry's deathgrip on content. Music wants to be free, geographically speaking that is.
If you're jonesing for some Japanese music via iTunes there are work around. Give something like jbox a try.