Rather like the phrase, English teacher, Chillum, isn't it? Is that a teacher born to English parents or a teacher of any nationality whose speciality is teaching the subject, English?
We call it that simply because it is made to a French recipe, the soup originated in France.The French call it soupe a l'oignon.: they don't need to identify it as French because to them it's just onion soup. It's a 'French' soup made with onions.