For me it appears highly genre-correlated. High percentage of science fiction books come with a small statement "this book is drm free on request of publisher / author". Zero of my photography, music, computer science or graphic novels came with such a tag.

Yeah, Tor Books publishes without DRM, and they seem to be one of the bigger SFF publishers these days. John Scalzi, George R.R. Martin (though not the ASoIaF books), Robert Jordan, Annalee Newitz, Charlie Jane Anders, and a bunch of other SFF authors I recognize. I'm sure there are others, but all the once I've noticed have been from Tor.