Timothy Taylor writes,

When it comes to textbooks, all choices are equally “free” to the actual decision-maker, who in this case is the professor rather than the student. If students at the college bookstore could choose among equivalent books by price, the outcome might be rather different. Indeed, professors who are interested can get a few free lunches, a conference or two, and a few hundred dollars for reviewing intro econ textbooks.

When professors choose a textbook, they are spending other people’s money. If a portion of textbook costs were deducted from a professor’s salary, the results would be different.