It's arguable that someone should get a warning for insulting someone out of the blue. Maybe that's a bit harsh if it's the first time, but it could be a good measure if it's going to prevent the poster from doing it again in the future. Like I said, it's arguable.
On the other hand, the concept of someone getting a warning for reacting to an insult is completely absurd.
The person who reacts to a personal attack is not the one that should be targetted by the mods, this person is not the one that started the whole incident in the first place, and not the one causing trouble. Of course, the most appropriate thing to do would be to shake off the insult and report the post, but asking for all members to be that cold-blooded is a bit too much IMO.
If not, maybe I should go postal and start insulting everyone, I would get warned but it would be an effective way to cause some posters (the ones that would insult me back) to get warned too. Wow, what a way to treat people equally!
PS: the "his insult was worse than mine" argument tells a lot about who is bringing it up.