This is text from an actual email from an actual coursera professor to actual coursera students.
"First, I know that some of you want answers to the assignments. This is a seemingly reasonable request but very difficult to accommodate. Creating questions for the videos and the assignments has been the most challenging part of this new endeavor. Four people, including me, worked several months to create these. We believe our assignments are well thought out and reflect a good balance of conceptual and applied stuff. Creating the assignments online and then testing each one multiple times takes additional time. Due to copyright issues, we cannot simply give you questions from existing books, and I would not want to do that anyway. If this were a one-time class, we would have considered posting answers. It will however be very difficult for us to offer this class again if we have to keep preparing new sets of questions with multiple versions to allow you to attempt each one more than once. Handing out answers will force us to do that."