In the case of a web service, the offending person is storing the illegal content on the server and violating the TOS he signed, which protects the web service. Your personal computer does not offer the offender a TOS to sign. Even if you use a free wifi in a coffee shop these days you have to sign an agreement not to use it for illegal purposes. Like I said I would like to see a criminal lawyer look into this and make a statement whether you can or can't be prosecuted for this.