Because paid for books are already on amazon or Barnes & Noble, or what have you, and their respective computer science/programming sections — including an indication of how popular the book is, and often also reviews by customers.

I like that this is only for free books, since, as a poor grad student, I can actually do more than just window-shopping.

For-pay books are pretty well covered already. As a student, I can't always afford the books I want, so this sort of resource is useful for people like me. Free books authors also don't get as much marketing buzz as mainstream authors, so I hope this helps to gain them more recognition and support from the community that would most benefit from their work.