I'm afraid you are being naive. The fact that Penguin does not come clean with the fact that books bought from their site include DRM has
been discussed on MR before. In general, publishers control which books have DRM added--though individual authors can negotiate that with the publisher. I have heard that Sony adds DRM to all books downloaded from their site. Kobo has DRM free books, but the fact that they are DRM free appears to be only visible on the version of the store that one sees using a mobile app.