It's to cover the sites back. No one under 13 is allowed on here, due to it being a 13+ site, though nothing on here is really NSFW, because that's against the rules. And the 13+ thing is very common on internet forums. She knowingly broke one of the most known rules. I've done stupid shit before, I ain't gonna be a hypocrite and say how stupid it was, cause I'm also stupid.
But if they had left her knowing she is under 13, and her parents saw something they don't like, it ain't her that's getting in trouble, it's the staff.