How exactly does this work?

Now I understand "self-abuse" in the Catholic sense... boy, do I understand it... though that's probably TMI... but how exactly does this constitute the "sexual abuse of children" when you're talking about something done by an individual to themselves, involving nobody else, of their own volition?

USATODAY.com - Teen girl charged with posting nude photos on Internet:

"A 15-year-old girl has been arrested for taking nude photographs of her self and posting them on the Internet, police said.

...She has been charged with sexual abuse of children, possession of child pornography and dissemination of child pornography."