Damn, almost caught out.
I was about to side with Apple on this one, under the proviso that forums and ISP aren't responsible for what people post on them... and then (about 0.5 of a second later) reality re-asserted itself - Apple lost that safe-haven ground when it insisted on policing the Apple App store and forcing the apps to be approved before being published. Hence, they *are* (partly) responsible for what is being sold through the App store.