I guess, technically, by todays standard you can say the referees got all that nonsense with that last melee correct. They'll certainly say that.
There was a bit of "pushing" in the tackle that sparked the head butt as a reaction, so I suppose you could call that the instigating incident and therefore a penalty to Souths. And then all four players were "involved" in the melee and so all got binned. Boxes ticked, NRL happy.
But it was pretty minor incident and the referee was content to let it go. But Tom wanted to make something of it, so he headbutted an opponent and earned a penalty.
Fitzgibbon was "in" trying to stop Sam from becoming involved, so Sam hit him and headbutted him.
The problem with this one size fits all punishment approach from the NRL, is that the actions of Sam and Tom were really out of whack compared to the things that upset them, and yet they got the same punishment. Tom was rewarded with a penalty for gross over reaction.
And this has the potential to green light the sort of violence the NRL is trying to prevent, because you can "object" to anything minor and then get payback however you want, and not get penalised, relative to your opponents.
The only "punishment" from an overly violent action then, in this sort of case, is from the judiciary, which has its own consistency concerns, and doesn't bother Sam at the moment anyway.