I think, for some people, books have a higher importance than just "things". They are almost sentient, in a way. They are friends. So for some of us, they are deserving of a higher level of respect than just some thing that's "meant to be used", and therefore it is difficult to see people doing something that damages the book. I don't voice my opinion to people who dog-ear their books, I respect that everyone has the right to do what they want with their own things, but it does hurt me in a way to see a book "hurt" in that way. I feel the same about not keeping your books neatly in a shelf, holding a book "inside out", "cracking" a hardcover, etc. You have every right to do these things to your own books, but I have a hard time not being silently upset when I see it.