This is a good argument. I'll try to speak for others when I say that the problem is with professors who actively engage in predatory behavior to gouge students. I appreciate that's not the norm, but it happens enough to at least be perceived as a major problem.
The predatory behavior lies in identifying a group of people who already have considerable sunk cost in their education, and then using a combination of methods (such as decreeing a monopoly by assigning your own books, and creating scarcity by insisting on this year's revised edition) to force them to purchase material or potentially waste the enormous sunk cost already incurred.
You write "we" don't do that, so I'm assuming you're a professor who assigns, and maybe even writes, textbooks. I'm sorry if this whole thread is turning into a personal attack on you. For my part, it absolutely isn't meant to be that. It's an attack on predatory practices that (a minority of) members of all professions occasionally engage in. This time the profession in question happens to be university professors.
I think few people have problems with IP law in and of itself. We have problems with the weaponization of said laws to exploit those in vulnerable positions.