Tor is totally used for criminal activity. That doesn't mean it is inherently a bad thing, or that it is this guy's fault, but he can't completely wash his hands off it. If bad guys use the postal service, it's not the postman's fault, but he has to cooperate with law enforcement if they demand that.
I don't know about the US, but contempt of court is a thing in the UK at least. You can't refuse to submit evidence to court, including things like encryption keys or things only stored in your head - or face penalties including unlimited jail time.
Now, I get that this is the US so the arrest was dialled up to 11 and it seems all of this is extra-judicial - no court warrant etc. This is all very disappointing. But, to my non-expert eye running a Tor exit node is in the legal grey zone, and I guess you can't be too surprised when things like this happen.