I don't think it was their way of explaining their outfits (after all, do they really need an explanation anymore than Superman's on-the-outside underwear gets an explanation?) but rather of trying to preserve the show-audience cultural connection. Part of the show's appeal in Japan was little girls getting to see superheroes who wore school uniforms like they wore. For a North American audience, perhaps the term "Sailor Scouts" allowed little girls (and boys) watching who were part of the Girl Scouts or Boy Scouts to feel a similar connection. Just my speculation. It would be great if someone who was involved in the production/writing of the dub at the beginning would give an "official" explanation.