[hackermailman]

He has extensive posts on his reasons, but it's also used for a course so only letting other professors have the answers to allow reuse of exercises is another reason. Those Art of Problem solving olympiad books don't have answers either with authors claiming same reasoning and in their defense I did learn a lot figuring it out myself. Personally I too like gratification of solving something then seeing the answers and finding a different and almost always more elegant/clear proof to compare to mine.