Interesting that the chart also documents a real-dollar decrease in the price of recreational books. That also needs to be explained.
AFAIK college students today mostly buy used books. I wonder if used prices are going up just as fast. Maybe faster!
As for the professors, I think they are in a hard place. The publishers seem to be releasing new textbook editions very quickly. If the prof continues to assign the old edition, every student would be forced to buy used, and a few wouldn't like that. More important, there's a danger that booksellers, especially locally, would run out of the old edition. So, regardless of whether it is the best choice, there's a lot of reasons for the prof to go with the new expensive edition.
So long as the school owns enough reference copies to meet demand for in-library reading, I don't have a tremendous problem with assigning expensive books.