I think it's great and very generous of books to sponsor contests. The few contest accounts that I have scored were all with very good books, and getting credited was hassle free with all but one. (It's kind of embarrassing for me to have to make phone calls regarding freebies)
Maybe it's some "free money" syndrome, but I have NEVER cashed out a cent playing with contest winnings. My largest "score" was $250 with Wsex, and I happily posted up with them after frittering it away.