I agree that the normal/flying pokémon should be only flying, but I think that happens to reflect that those birds are common birds with no special atribute (opposing other bird pokémon that have a relation to an element such as Swanna and Pelipper for example, which are related to the water...). Tornadus, on the other side, is the japanese god of the wind (plus, it's not a bird), its only atribute is the wind, so it wouldn't make sense for it to be "normal"-type...