The idea, however misguided, is that the consumption of child pornography creates a market demand. If nobody wants the images, nobody will be compelled to create them in the first place thereby eliminating the actual abuse.
1) The causal link described remains a matter of dispute, with evidence supporting both sides; a plausible argument can be made for the opposite case that wider access to child pornography, and making more 'efficient' use of existing images, could quell demand for the creation of new images, and thus reduce new abuse cases;
2) Legislation prohibiting child pornography often extends its reach beyond cases of serious abuse; in many countries it encompasses 'softcore' images, even semi-nude images if they are sexually suggestive. These kind of images (the kind that you would find on /r/jailbait over on reddit) are often created by the subjects themselves, and one could argue that their creation caused no harm to anyone.
1) I think that's fair, but legal arguments are not always logical. I'm reminded of the Illegal Numbers topic that was on here in the last day or so.
2) Many times I think people forget what laws were originally intended to resolve and they get updated and twisted throughout the years. Not unlike the drug laws. Who even knows why they are illegal anymore? Especially when so many other drugs with the same social/economic problems are perfectly legal.
