This ties back to why opponents of copyright as such call it "imaginary property:" there is nothing about the copy that makes it any worse than the original. We can't recognize a copy because then it isn't a copy, in the information theory sense.
Meanwhile, in the physical world, a knock-off handbag can be easily identified as such by airport personnel. This is the world these pro-PIPA politicians live in. Child porn, online as offline, is inherent in the work. You don't need supplementary information to decide if something is child porn. Online, the difference between a copy and the original is in some people's minds, not anchored in reality.
Meanwhile, in the physical world, a knock-off handbag can be easily identified as such by airport personnel. This is the world these pro-PIPA politicians live in. Child porn, online as offline, is inherent in the work. You don't need supplementary information to decide if something is child porn. Online, the difference between a copy and the original is in some people's minds, not anchored in reality.