Do you know of any mundane teachers who teach people how to bend reality? (particle physics et al, I know, I know... but still) Yes, mundane and arcane knowledge are both extremely important, and sometimes they intersect, but magical teaching carries extremely heavy spiritual responsibility that some believe is recorded and dealt with in the spiritual world. Some venerable old Craft traditions even believe there are guardians of the magical realm that record every single magical act a witch performs and assists in balancing the cosmic scales. This is not to be taken lightly.
CE, forgive me but I don't see how your last sentence is relevant to the question. How does a person's sense of responsibility about teaching mundane things enter into the ethical question of charging for magical teaching except to show that some mundane teachers may teach irresponsibly? That's an entirely separate subject.