Most of the professors I've had taught the course entirely with free materials. I think they went out of their way to avoid having students spend money. A top-down policy forcing all students to buy textbooks sounds like a way to take control away from these professors in order to further enrich the school and the publishers at students' expense.

I don't view textbook costs as a "dark pattern", they're an honest signal from the professor to the students. A professor who makes a $250 book mandatory and then barely uses it sends one kind of signal. A professor who goes out of their way to hunt down (or even scan) and provide free PDFs for all required readings sends another. Students receive these signals and they're reflected in professors' online ratings. To the extent possible (which is a large extent for some highly flexible programs, such as mine), students will go out of their way to drop and swap classes to avoid unpopular professors, further incentivizing faculty to keep these ancillary costs down.