How do you suppose one should make sure the exit node you run won't be used for sharing of child porn? And how is he automatically an idiot for running a TOR ex

How do you suppose one should make sure the exit node you run won't be used for sharing of child porn? And how is he automatically an idiot for running a TOR exit node? All node operators run them knowing they could be used for illegal activities, and that it could bring them some legal trouble.