I agree on the general point, and don't think that he should be found guilty if he was just relaying traffic. However, I think that you have to be very careful about analogies like that because there's a wealth of subtleties. I'd like to investigate a couple of them here.

Most of us would probably agree that convicting the post office would be crazy, yet there's an entire continuum of package delivery services including the government postal system, private delivery services like UPS, professional couriers and asking your friend Steve to drop something off on his way home from work. If Steve gets caught delivering a package that contains drugs, his claim of being an unknowing "relayer" would likely be viewed with more suspicion than the same claim coming from a UPS truck driver.

That reasoning makes sense to us because we know how package delivery systems work, and have a vague idea of the probabilities involved. However, Tor is new and niche enough that you can't rely on an arbitrary person (or even a police officer or judge) knowing enough about it to make those same judgments. They don't know whether Tor is more like Steve or the postal service, and it'll take time before that knowledge is assimilated.