It's going to largely depend on the publisher. The publisher, not the platform, controls whether the book is sold with DRM or not. The majority are, yes, although the one genre where that's generally not true is Sci-Fi. There was a time, at least, when most of the major sci-fi publishers opted not to sell their books with DRM, because they thought it was an abuse of technology. Not sure how much that still holds true, but publishers like Tor still sell all of their stuff DRM-free.