The Bonus tracks are offered to persuade Japanese consumers to buy the expensive local product in an open market.
increasingly this is happenign here now in Australia since the industry was butchered (sorry, de-regulated) a year or so ago...
And guess what - prices didn't go down with deregulation...how about that!!!

Also - from talking to the musicians involved, the Japanese are increasingly using bonus tracks to strengthen exports and therefore sales.
But the exports don't go towards domestic sales results and therefor chart results....

Just a few thoughts...

Regards,
Andrew.