This is the only video I could find by AKB48 that is nude. They are very similar to Korean girl pop bands like girls generation.
I think the nudity is merely a stunt. Whoever is in charge of them, probably pervy old Japanese men, wanted more money and told them to do it to cause controversy, increase ticket sales, and get their band name out there more. All of which create more money. More money generated means the person(s) in charge of them will make more even if the band members actually don't. I don't they're playing nude is a main stream thing there.