Mr. Driller (DS) slights US consumers

Apparently, US & Japanese consumers will be getting two significantly different versions of Namco's Mr. Driller for the Nintendo DS.  In a nutshell, the Japanese version will allow gamers to play multiplayer battles with only one cartridge, while the US version forces everyone to have a copy of the game.  The Japanese version will also feature the "Dristone" mode (we have no idea...).  At any rate, these unconfirmed reports leave many of us wondering, why?

[Don't forget that the DS is not region encoded, so US gamers should do just fine with the Japanese version, text aside]