Not only the immediate perpetrator commits a criminal action, but also anyone who appoints someone to carry it out, or anyone who otherwise contributes to the completion of said criminal action.
In other words, it's a form of accomplice liability for criminality. It's pretty standard to name criminal accomplices liable for "aiding and abetting" the activities of others, but it's a massive and incredibly dangerous stretch to argue that merely running a Tor exit node makes you an accomplice that "contributes to the completion" of a crime. Under this sort of thinking, Volkswagen would be liable if someone drove a VW as the getaway car in a bank robbery. It's a very, very broad interpretation of accomplice liability, in a situation where it clearly does not make sense.