If he participated in it, and his students participated in it with his knowledge (which we know to be the case), then it can not be as black and white as it being flatly prohibited. But then...things rarely are in Japan - Western thinking tends to like things a little more clearly defined.
Well, also...just because someone did something, doesn't mean it isn't illegal here and now (or wasn't illegal then and there, for that matter). Sometimes people get away with things; that doesn't mean you will if you do the same. "But he
did it!!!" doesn't even work on a first-grade teacher, much less cops and DAs.