Because I made a personal decision that I wouldn't torrent anything but open source stuff. If I'm going to format shift, I will buy the item. The author deserves the money for continuing their efforts.
I don't disagree with your decision to not torrent, but whether the author gets any money from format shifting is determined by each author's contract for each work.
As much as I hate ereaders, there's an excellent example of why ebooks are a good thing.