Its a genuine concern for miners and people who use a wallet which downloads the full blockchain instead of electrum etc. This is because there is that 1 actual pic on the blockchain (not the links). These people will now have a picture of child porn on their hard drive. Possession of a copy of a child porn is always illegal no matter what. It is one of the few laws where possession establishes the entire crime and not intent. For instance if a child you dont even know randomly sends you a nude pic you are automatically guilty.