It could be due to the specific content made especially for Softbank mobile users provided by companies like Yahoo Japan (subway/train info, restaurant info, etc), or the Japan specific MMS system implemented, or a number of other items. I can’t see why this content and/or technology can’t be adapted to the standard iPad.
However with that said, there has been talk about the industry changing to allow the consumer the choice of mobile provider and offer unlocked hardware, but this change is coming woefully slow.

The point about content is a canard. It would be a relatively simple matter for Yahoo Japan to block access to their content for anyone attempting access from any point other than a Softbank SIM or WifFi access point.

For too long, the Japanese mobile carriers have promoted their badly-designed, dysfunctional proprietary walled gardens to the point where many Japanese believe that these are “the mobile Internet”. The iPad and iPhone offer a chance to break away from this mentality - hence they’re dangerous to the content monopolists. In the meantime, marketing for phones in Japan will continue concentrate on the number of colors offered, and the bells and whistles, rather than on true functionality (let alone ease of use, which is the real differentiating factor for the iPhone and iPad).