See I think Greenberg has a point. Whilst I certainly think legality of a player's actions should be given significant assessment when deciding upon a punishment, I think the actions themselves and how wider society perceives them is just as important. Case in point here with Roberts - two drunken incidents, yells at a barmaid, given short term ban by police. Society definitely will frown upon those actions, but they don't really carry any shock value - plenty of people themselves have been in a similar situation, or know of somebody who has. Compare that to the Pearce incident - sure no cops or violence, but then how many people know of someone who has gotten trashed then pretended to f**k a dog? His actions carry more shock value.
When push comes to shove, the NRL (like all organisations) needs to consider the implications for his own brand when a player misbehaves. Ultimately, they are going to have to come down harder on someone who's actions are more widely condemned by society to minimise damage to their reputation. Not condoning Roberts' actions either btw (personally think he should have been given a week or 2), but rather easy to see why the incidents aren't comparable.