Mostly people say GCN spins backwards because it isn't being pirated yet. The panasonic GameCube-Q reverses directions when you put in a DVD or CD? Bah.
People are confused, but there is a nugget of truth floating around amidst the general cluelessness: GCN reads discs from the outside in, rather than the inside out. None of the systems spins the discs backwards, as you pointed out.