There is an difference in having no incentive to keep costs down and having an incentive to keep costs up. Some professors are simply indifferent to their students' plights. As far as these professors are concerned their students are just an inconvenient bleep in their oh-so-important life. Extracting money from them is not on their radar.
The vast majority of professors don't write textbooks. In fact there are probably a lot more profs who do care, and assign open books, or no books at all, than professors who make a living writing textbooks.
