It probably has to do with how word of this book and the book itself was practically viral among parents and the content was good enough to deem buying in hardcopy for a lot of those pirates. I was under the assumption that this is generally how the piracy success stories work. A lot of people pirate something only to discover that they really like it, and they'll buy it to make up for the piracy.

I'm not even a parent, but I saw the book on facebook through a friend-with-kids and was howling the entire way through. I preordered a copy on Amazon immediately after finishing reading the book and I bookmarked it for future reference for gifts to new parents. If it wasn't for the book being on facebook, I never would have heard of it and I probably wouldn't have bought it if I only saw the cover or a page or two on Amazon or at a physical bookstore.