My guess is they wanted you to pay the higher CS tuition to take the CS classes.

Math departments also tend to be a lot more lax in my experience. Case in point: I got sign-off to take a 300-level pure math class without its 300-level prereq. As well as just replace one required course for another to graduate.

Students aren't the business most universities think they are in.

Academia being inelastic and refusing to adapt in the face of market forces.