In a more (but not completely) serious way, showing that it doesn't yield the maximum profits. Giving away info can be the end by itself for some, so they can't be helped.
Others hope to "advertise" their intellect in the hope that they get selected for a well-paying job, for example. It's like offering a free sampler. In this case, it suffices to appeal to rationality, and to point out that restricting the free give-a-ways the people are forced to pay for what they want.