That is because there are several apples in a tarte aux pommes. Now you could argue that there are several onions in a soup of onions, but is has to be soupe a l'oignon in the singular. It is usage you see, like the great French classic sandwich au jambon. You just do not argue. French chefs have knives and they cannot be bothered. Well that is just French grammar. A lot of rules and even more exceptions :)