Not that I've ever encountered it in the wild, but I feel like pointing out that publishers can opt-out of DRM on Amazon as well. You'd recognise them by this in the description:
> At the Publisher's request, this title is being sold without Digital Rights Management Software (DRM) applied.
I only know about it because of Cory Doctorow, never seen anyone else that does this. Heck, even re-packaged public domain books contain DRM for some inexplicable reason.
Kobo also does it. They tell you in the eBook details. It's the publisher's request. Publishers like Tor, O'reily and Baen go DRM free. If the re-packaged public domain books don't request it then on goes the DRM.
> At the Publisher's request, this title is being sold without Digital Rights Management Software (DRM) applied.
I only know about it because of Cory Doctorow, never seen anyone else that does this. Heck, even re-packaged public domain books contain DRM for some inexplicable reason.