This article never comes out and says it directly, but it sort of has this tone like this is some sort of unusual thing. Hardly. You'd be hard pressed to find a successful web comic running for several years that hasn't put out some sort of book, or some sort of tangible, purchasable artifact. It's a great way to support your favorite comic while getting something nice back, if donations are too hard (or in some cases, not accepted).

(No disrespect to xkcd, and kudos on getting the NY Times placement.)