Teachers wield a lot of power. I noticed this when I was teaching, and see it more now that I'm tutoring. I can give a student something that will help him or her to understand something, but if the teacher didn't assign it s/he may or may not take it seriously. They don't even go to the freely accessible web site that the book references unless assigned.
I don't know how the hell I'm going to get them to pay for mine. They will pay for me to tutor them, though, so I know there is a way.