Simply because I like when I purchase classics (not covered by copyright any longer) that I have the ability to choose the book based on criteria I cannot usually factor in.
I can opt for quality paper, for a nice and well-spaced font, for good-binding, for a nicer jacket, for a specific translation.
I like that there is the potential that I could purchase this book and be able to choose such things, that I could say "I'll pay this guy over here a little more as he's invested in the quality of the medium.".
I was very happy with the contents of Coders at Work for example, but disappointed with the paper selection and choice of font. I would have happily paid more to have a nicer package, so I love authors who would permit multiple companies to do this.