Allowing a standard, open format to work on the Kindle would make sense for users; Amazon currently chooses to disallow it. Imagine having a DVD player that could only play Amazon-formatted movies and not standard DVDs.
Buy an eBook "from Google's eBookstore, you'll be able to read it on your desktop PC, your iPhone, the Nook, the Kobo eReader, the Sony Reader, or a garden-variety flip phone — just not your Kindle."