It's no longer their property after they have sold it. If I buy a book, then that book is my property and I can take a Sharpie and mark out any words I don't like. I can draw Hitler mustaches on Hillary Clinton's photo on the dust cover. Where in the law does it say I can't do that? Shouldn't I be able to pay someone to do that for me?
Apples and oranges, you can deface a book, but what you can't do is take a copyrighted work like a book and make changes to it and then sell it.
Its pretty simple.
You can burn you book, tear it up, use it for a placemat and eat off of it. It's your book. You can hand YOUR book to someone to do it for you. But you can't buy one already butchered unless the author ok's it.