I've taken some time to consider this issue myself. As other posters have noted its impossible to stop in a system where data is transmitted. I prefer living in a world with free communication (with a channel for criminals) than living in a world with restricted communication. The true crime here is child abuse, not the sharing of data. We don't say people aren't allowed to associate in person without a government minder because they might be swapping photos of child abuse.
I see these kind of services in a similar light to safe deposit boxes at banks - where you also have near-complete anonymous storage. I doubt any bank manager is losing sleep because child pornographers could be using his service.
The biggest problem with this mindset is proportionality at the moment - bank safe deposit boxes have much more public awareness than these services, and have thus gained wider use (and acceptance).
What if you walked into the bank and there was a line of people with sticky pictures of dead kids in their hands waiting to enter the safe deposit box area. Would you come back?
Here's a thought...
Put on your foil hat for a second. Let's say you were a government and you wanted your citizens not to use systems like Tor and FreeNet. Wouldn't flooding them with extremely disturbing porn be a great way to make sure these systems weren't used by anyone except CP wankers? Wouldn't it be a great way to get people to go along with outlawing them?
This is a technical problem. When you think about it like that, it becomes obvious that this is a vulnerability in the security/crypto sense. I'd state the problem this way: these networks are trivially vulnerable to a particularly devastating social engineering spam attack that renders the network virtually unusable by most people. Call it a social DOS attack.
Edit: I believe I can state the problem succinctly:
Design a darknet/freenet network that is anonymous, uncensorable, and yet is not trivially vulnerable to social engineering DOS attack.
This is actually a really interesting point. However, its clear that absolutely any method of communication is vulnerable to the very same "social DOS attack," especially if we're imagining it to be perpetrated by the government.
Personally, I've spent quiet some time on the .onion network and never been ambushed by child porn. Don't you think your problem is solved on the .onion network the same way it is on the clearnet: with moderation on a website-by-website basis?
I certainly see links to it fairly prominently displayed on the Tor onion network, which I know would drive off at least 75% of users.
What I'm saying here is that there's an interesting unsolved problem and that this problem might be the thing that's blocking the adoption of these technologies.
It's also a critical mass problem. I don't think you could run that sort of attack against the Internet because it has over a billion users. Once the network reaches a certain mass, it becomes far less of a problem. The problem is that CP-wankers (and possibly attackers) instantly colonize darknets, rendering them quickly polluted before they have a chance to escape their nascent phase. Like I said in another post: you could apply game theory here.
Regardless of the plausible deniability of the contents,
from a legal standpoint, if a court issues an order to divulge the contents and ownership of the box, the owner of the storage resources (banker) can comply, whereas the owner of the encrypted anonymous storage network node cannot.
I'm not going to make a blanket statement that this is a good or a bad thing, since it largely depends on what's in there (the technology itself is neutral); and some jurisdictions might define as depraved indifference (minimally) or facilitation (maximally) should the contents be illegal.
Also I can easily foresee that being unable to trace the provenence of data stored on one's node could put one in a difficult position to assert it isn't one's own, when possession is usually all that is needed for criminal liability.
The banker can cover the ownership case, and hence his backside.
As far as public/private goes, it seems this network from what I read, maintains an opacity shield with regards to contents, but is peer-to-peer storage. So in that regard, neither the safe deposit box nor the storage network are "publicizing" anything, per say.
I know something - and that's how little you know. For one, if you did know, you'd be admitting to a crime.
But more-over, even if you investigated Tor exit-node traffic you can't know if any porn you might see was actually being traded or merely continually transferred by repressive regimes to have an excuse to ban anonymity providing services.
You suspect a lot, and have reached some conclusions, but you don't know what it is, let alone who if anyone is doing it.