Back when I was in college, many international students from e.g. India reported being able to buy exactly the same textbooks (albeit perhaps in paperback or lower quality printing) for ~1/10 the cost back in their home country vs from the campus bookstore. It was the pricing offered by the publisher, and there is an economic term for this, I forget, when you offer the same product at different prices to maximize your market share. But there were also legal restrictions on importing those discount textbooks back into the US, specifically to prevent you from reselling it at a higher price but less than the US price.
I suspect that the photocopying of textbook chapters by professors for distribution to a class as part of required reading is a highly abused practice, and that publishers (if they were aware of it) would be within their rights to demand payment of some sort. In many cases if you want a single chapter from, for example, a scientific textbook, you can purchase it online from the publisher for prices ranging from $10-40, and for distribution to a class they would expect some sort of per copy charge (different from the cost of photocopying that the campus print shop would charge back in the day... is there still such a thing??)
The one time I recall engaging in copying big chunks of textbooks is when the book was out of print and I got it via interlibrary loan and only had a few weeks before I had to return it. It was probably still illegal to do so.