All these examples of buying paper books with silly DRM equated restrictions miss one major point in the comparison ... all the eBook websites (I have looked at) state quite clearly that the books are DRM protected, and will only work on certain readers, and thus all these book shops would have a sign in the window stating the terms and conditions under which you are allowed to read the book you buy from them. If those conditions didn't suit, you would either buy it anyway and google for a way round the restrictions, or you would find an alternative place from which to acquire the book which doesn't have restrictions.
I think phrase in point that is generally missing in discussions of DRM is
caveat emptor: you know exactly what you are buying, if you don't like it, don't buy it!