None of the commenters, including Dorn, appear to understand "student motivation" and how "rewards" affect motivation or lack thereof. The result is a lot of ill-informed opinion, i.e., the typical state of affairs in education opinion journalism.
Here's a forty year old excerpt on the topic from Your Child Can Succeed:
A child who is told to do his arithmetic worksheet has choices. He can either do the worksheet, look out the window, draw a picture, or belt the little girl next to him. Operant Psychology would hold that if you want the child to choose one of these actions over the others, you have to make that one more rewarding (or less punishing) than the others. The value in making the desired activity rewarding (rather than less punishing) is that if the child learns that working arithmetic problems is "rewarding" he will tend to work on arithmetic problems even when he is not rewarded. If he is taught that every time he doesn't do his arithmetic problems he gets clobbered, he will learn a great deal about what happens when he doesn't do arithmetic, but very little about the rewards that may be associated with doing arithmetic.
Give the kid a reason for doing what you want him to do. Set up a contingency so that if he performs he receives something that he wants. The only way he can get the payoff is to do what you want him to do.
Some children do not work for the joy of doing arithmetic. By using payoffs to get them started, the teacher can systematically build up "motivation." At first the child is interested only in the specific payoff--the candy or the extra recess. As he learns, he receives other payoffs, such as praise for good work. After a while he learns to treat the payoff more as a symbol of his competence than as an end in itself. And he learns that the work itself was perhaps less than fun but certainly not punishment. Finally the child will be willing to work for nothing more than the praise and sense of achievement associated with performing well.
There is, of course, much more to student motivation than is presented in this short excerpt. This issue is far more complicated than the commentary would lead you to believe. Check out this series of posts on How To Effectively Manage a Classroom from an actual classroom teacher who's been successful motivating recalitrant students.