He wasn't searching for it or knowingly distributing it. The way Tor exit nodes work is that you're hosting a machine that lets other people on the Tor network communicate with the internet. You're essentially routing a portion of the entire network's traffic through your machine. You can't really control who is using it or what it transmits at that point.
He got punished because somebody else shared CP, using his equipment to do so. It's like being jailed for having your car stolen and being used to hit a pedestrian.