DRM-Free eBook sellers

Cory Doctorow, via Boing Boing:

" Amazon, Barnes and Noble and Kobo were all happy to carry my books without DRM, and on terms that gave you the same rights you got when buying paper editions. Sony and Apple refused to carry my books without DRM -- even though my publisher and I both asked them to."

I wonder why Apple responded this way, given their stated position on DRM-free music. It would be interesting to know more about the negotiations.