Probably, the reason is because, our laws change so often.
I do know for a fact: a woman was arrested for having non-sexual photos of her children in the bathtub.
and, a man was arrested, after having his computer worked on, in Best Buy. He had ONE non-sexual photograph of a naked child.
This hysteria is fueled by the media.
If the girls are nude only and not "doing stuff" with each other, then it should not be a big deal. I haven't seen the photos, so I don't know what they were doing in them. Were they just naked and standing there or were they doing X-rated stuff? That's the difference between nudism and child porn. Nudism or naturism is non-sexual nudity and being one with nature. Porn is quite a different matter. So, again, what were the girls doing? Just being naked or doing stuff with each other?