Grading on a curve is retarded to begin with. If the professor only gives an exam about what (s)he taught, then its up to the students to pass or fail. If the professor knows from experience that the mean score on exam will be low, then they can define a score of, say, 40% as minimum passing grade, and, say, 80% as an 'A'. That way it can't be gamed. As long as the target isn't moving it can't be manipulated.