You put years of your life into learning, researching, writing, and editing a book so that students can benefit from it. You sell it at a price that is reflective of how much time and effort it took to write it (textbooks are not novels, you know). Then some cheapskate kids decide that your work isn’t worth the paper it’s printed on, simply because they perceive it to be expensive compared to their midnight Taco Bell runs. But alas, these same students still want to reap the benefits of reading it! So what do they do? They just find a way to rip it off - because of sticking it to the man, those big evil textbook companies! And then they see nothing wrong with that. But what about the author? Is he just left out in the cold?

Now let’s go back to the situation at hand. The professor in question may not be the author of this particular book, but he has likely written something or worked with others who have. And likely feels very strongly about kids who try to get something for nothing because he is intimately familiar with the kind of effort it takes to write a textbook. And as such, he wants to ensure that others receive fair compensation for the labor that goes into making something as comprehensive as a textbook. He is choosing to take an active role, rather than passively looking the other way. This way, at least some deserving author will benefit, even if it is not the one originally intended.