Should law enforcement just say "Oh, you have an open Wifi hotspot/you're running a Tor exit node, I guess this is a dead end, thanks for your time"? Investigating (including seizing equipment) to determine whether the traffic in question actually came from the Tor network seems like a reasonable idea. Otherwise, anyone running an exit node would have a blank check to do whatever they'd like and simply claim it was just traffic they were passing through.
I know that people like to pretend otherwise, but as long as there are crossover points between the Tor network and the public internet, there will be non-anonymous people in the real world who will have to deal with things like this. "I'm running a Tor exit node" is not (and should not be) a magical pass that makes you immune to any kind of investigation.
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.
I know that people like to pretend otherwise, but as long as there are crossover points between the Tor network and the public internet, there will be non-anonymous people in the real world who will have to deal with things like this. "I'm running a Tor exit node" is not (and should not be) a magical pass that makes you immune to any kind of investigation.