You're rewarded for your presence with a grade and a step closer to a degree. When you don't go to class and let others take notes for you, or purchase them from other source, you aren't doing your job. You aren't even guaranteed that the notes you get will pay off at all. You are authorized as a student to your own interpretation of the lecture in your notes. You aren't authorized to sell these notes for your own profit.
Some professors choose to have their lecture notes be sold through other locations.
In this case, they've given their permission for their ideas to be shared that way, just as if they've chosen to have their notes posted on a Web site. For doing the work, it's their right to choose how it gets to their students. Hope you've been taking notes. And that you don't sell those notes for your own profit. Keith Thomas, Tom McHenry, Melissa Davis and Laura Pelner.