I've canceled before and kept Fast Passes, but they were for the Safari, Everest, and Dinosaur, so not really anything for Disney to get mad about. I didn't book for the window, it was my wife's birthday and she decided to stay off property at a much nicer hotel for around the same price. I was shocked that Disney had no cancelation fee, bet that's coming too.
EDIT: Now that I think about it, the lack of FoP on the first day played a role in our cancellation. The three we ended up getting are all typically available at the 30 day mark, and probably the day of in most cases (we were there on a weekday in October). We were upset that after well over a year, it was still impossible to get FoP passes despite having a reservation and checking on day 60, the first day we could. Coupled with the added parking charges, we decided it wasn't worth it and stayed off property at a nicer hotel with free parking. I will say that had we scored FoP passes, we probably wouldn't have cancelled. Other factors played a role too, but I think we would have just toughed it out and kept the reservation. Without the window though, the added parking and less features (it was All Star Movies), it wasn't worth it to us.