A lot of teachers like to hide the previous week's sections just to clean the page as well. I could also see where a final quiz might be a conglomerate of earlier quizzes and a teacher might not want her students going back to look at the questions. (I know, why not?, but I am sure they have their reasons!!).
Whatever their preference, it does not make sense that the quiz module is the only thing that does not allow a grade to show for a hidden activity.