LOL, JR doesn't have the right to punch anyone in the face, that's absurd. And you can't get sued for libel or slander if what you say is the truth. Also, NBC has a large and ugly history of having employed men who are accused or found to have sexually harassed their co-workers, so one could expect them to err on the side of caution in that regard.
But the bottom line is, NBC could very easily have fired him because he wasn't very good at his job, people didn't miss him on the broadcast and their ratings showed no drop without him. In other words, he was easy to replace with anyone else who could utter a complete sentence, but without the harassment baggage that JR has. Go to the main board thread on this. Half the posters say he sucked, zero of them say he was "good" or "I liked his bits".