I really don't think this is an issue. If Oprah opens some of her fan mail and there is child porn inside, would she get arrested and go to jail for being in possession of child pornography? What if someone turns in a paper to a professor and it has illegal things in it? It would set a dangerous precedent I think.

I don't know the answer to this, but what if someone ran a Tor node and illegal things were sent through it? Same with proxy and VPN services. I'd think that VPNs would be non-existent if this was the case.