"Furthermore, even in the brutal-rape case, the child is not harmed when a third party makes copies of the video."

What about the privacy rights of the victim? The argument could easily be made that a child can be traumatized all over again when finding out years later a recording of their abuse is spread far and wide on the internet and they have no chance of ever curtailing it. Whether it's feasible to prevent that (as with any other information) is another question, but copying images of child abuse doesn't seem that victimless a crime to me.