From what I've heard thus far, a lot of the issue stems from 'jailbait images' posted on blogs. It's tough because those are in a bit of a legal grey area. 'Jailbait images' are photos of preteen or teen girls (or occasionally boys) in swimsuits or revealing clothing taken from legitimate commercial sources (like advertisements for clothing) or private sources (family Facebook accounts, the kids' Snapchat, instagram, and twitter accounts for instance) that a non-pedo would find innocuous. But, pedophiles save them and share them in inappropriate contexts while making inappropriate comments.
The problem is the context that makes it inappropriate, not the image in and of itself, so the law can't do much because there is a legitimate non pornographic purpose of the images (advertising, self expression, families posting pictures of events) and restricting it would paradoxically sexualise it more. It was not created to solely arouse, so it isn't classified as obscene or pornographic. Sometimes, the images themselves come from other Tumblr users (remember, teen girls are a big demographic on Tumblr) posting pics of themselves and then getting accosted by creeps, which is horrible and most certainly sexual harassment.
So, the safest bet was taking the app off the store to figure out what to do about these jailbait images, because the law may not leave much recourse.