I'm gonna disagree Mop it up, I think it is more because it is easier to handle since they didn't build the Wii Shop infrastructure with the DSi in mind. Imagine all of the potential hassle and confusion in creating a way to sync all of the accounts up. You could accidentally sync the wrong accounts and then people are spending other peoples points. If they had developed everything from the start with the intention of a seamless and integrated infrastructure akin to the XBLA setup then it wouldn't be as much of an issue. As has been pointed out, Nintendo programmers don't create systems with a lot of flexibility and adaptability in mind so it would be extremely difficult or impossible to feasibly reprogram all of this stuff to work together.