To be fair, used textbooks have highlighting in them. I gave up on used books in law school because the existing highlighting and margin notes were often wrong and distracting, and got in the way of my own highlighting which was a big part of my workflow.
I'm sure this was part of his reasoning. Now that you mention it, I'm not sure I would buy one from Amazon for this reason. I bought used textbooks from the university bookstore all the time when I was in school, because I could flip through them and find one that wasn't badly marked up, but you can't exactly do that with an online retailer.