A complete stranger knocks on your door and asks you if it's ok for FedEx to deliver an unmarked parcel of DVDs to your house, and he will collect them from you later.
You do not know his name or address. The package arrives. You open it and examine the DVDs, but find that they are encrypted. You pass them on as agreed.
Next day the feds knock on your door, saying they have a tip-off that some child porn, bomb-making manuals and stolen bank account details were delivered to your address. You tell them exactly what happened. They seize your laptop and search it for said materials, but find nothing incriminating.
Have you committed a crime?
I hope not (in both cases) but at the same time I think if the tor exit node operator is protected from prosecution for illegal material passing through the node, then the person passing on the mysterious package should have exactly the same protection.