That's why they raided his house.
> Would they raid a Starbucks if someone used Starbucks wifi to distribute CP?
Of course they would. Obviously aiming to try and catch the perpetrator in the act, but also at least to get a good look at the network configuration.
"CP came from this IP endpoint" means crime is being committed using this network. It's perfectly reasonable for them to go and investigate. And as others point out, them knowing how Tor works makes no difference - otherwise people making/distributing CP would host their own exit nodes with the sole purpose of being able to point them out to police and tell that CP it's not their traffic.
They didn't jailed him, they checked if he is in fact involved - and finding no additional evidence, left him in peace.
Would they raid a Starbucks if someone used Starbucks wifi to distribute CP? No, they would need more evidence that someone at that Starbucks was doing it and not just someone who bought a cup of coffee there that day.
Tor is open wifi, the IP of the exit node doesn't provide evidence that the person running the exit node has anything to do with the traffic.