Child pornography is dissimilar to copyright piracy. Child pornography is in a legal category unto itself, at least according to United States case law.
"13yearoldinabikini.jpg" probably cannot be classified as a pornography. Most of the things they banned were probably legal (I can't imagine there really was a true child pornography subreddit). But I understand having these subreddits around was a slippery slope and some content might have been over the edge. And as was already mentioned, it's hard work to control it post by post. Especially when some askreddit post accumulated the infamous reddit-hivemind and they went on reporting every single post in "targeted" subreddit (whatever the content).
