> Books enter the public domain.

...and then they get re-packaged with DRM on Amazon's store, mostly because people uploading public domain books on Amazon have no idea what they're doing.

> Project Gutenberg and others produce DRM-free versions. Many academics and people who wish to share their knowledge also publish works DRM-free, sometimes under permissive (copyleft), licenses.

You can read DRM-free stuff on a Kindle already, so that's not particularly relevant here.

> The fact you see DRM as the norm and non-DRM as “a unicorn” that “doesn’t exist”, is mildly sad.

When every big publisher is doing it, it is the norm. That doesn't mean there doesn't exist any book publisher which doesn't do this, but the vast, vast majority of the books actually sold today contain DRM. We don't have to like that norm, but pretending it isn't one is just denying reality.

This.

While lots public domain books are on Amazon's store, most of those books are not free, both in the sense of "free or charge" and "DRM free". A lot of literature classic are released by a major publishing house with foreword and annotations, which to be fair, are copyrighted works and provide value. And they cost a bit of money. The "real" public domain versions provide by Amazon are barebone. Those versions are often good enough for many people, but you don't need to get them from Amazon in the first place.

In other words, public domain or not does not have much to do with DRM-free or even Amazon.