Sorry, but this is an incredibly dumb argument(no personal offence meant). If you(or anybody else here) went on 4chan at the moment when somebody happened to post child porn, your very first reaction would be to wipe your history, so you could not be charged with possession, even though you did not want or meant to see these images. If just merely seeing them(so technically - downloading to your computer) were not a crime, more people would actually go out and report them to the police, which increases the chances of actually catching the producers of these materials.

And as it has been mentioned before - possession of CP was made a criminal offence back when the only way to get it was to pay for it. Nowadays, you can easily find it online, for free - without knowing who created it. So making it illegal is by no means helping anyone catch producers of child porn. It's not like making cp legal would suddenly make it ok to rape children.

You might think it is dumb, but it has repeatedly proven itself to be an effective tool for piercing criminal networks.

Police don't need people to report CP on 4chan--they can easily monitor 4chan themselves. (or any other public site)

It's useful when they find thousands of photos and videos that are recent and not publicly available, on a person's HD. Since possession is a crime, they can plea that person into turning over their supplier, then roll him too--hopefully traversing the network to find the person who is actually making the photos and videos.

Maybe,but it has also proven effective in putting people who are not child molesters in prison for child porn. They should be helped,not criminalized - it's people who actually harm children who should be put in jail for the rest of their lives. And years ago you could only buy it from others - and that's why this technique of making possession illegal is so effective against drug dealers. If possession of drugs is a crime,then you can always demand to know who is the dealer, from the person doing cocaine or whatever. The problem is,that this doesn't really work against cp - sure, maybe 20 years ago when to get it you had to buy physical pictures from some guys in a back alley it made sense. But what are you going to get nowadays? Hidden wiki's address? If you go on TOR there's tons of this stuff, and police cannot monitor it effectively,and cannot do anything about it, so it's unlikely that persecuting a person viewing this material is going to get them nearer the producer.

It also works against anonymous criminal networks, because the "turned" defendant can take actions that will result in the disclosure of the identity of the other anonymous participants. See: Sabu.