I read the IRB Judicial Rules, and It seems that Rhodes got the maximum recommended penalty for his offence, and not the top end penalty of 10 - 52 weeks.
His offence was sickening to watch and very dangerous, and its great that the judiciary have come down hard on this type of dangerous play ... it was interesting to hear Justin Marshall (I think) in the commentary just after it happened, say that players are actually trained to grab the player under the arms, and twist their bodies out of the rucks in a kind of Judo throw, so, giving Rhodes the benefit of the doubt here (and not condoning what he did), it looks like it was one of these clean outs gone wrong.
This is probably why he escaped the high end category of punishment.
Piutau probably should have received the low end penalty of two weeks for striking with the hand IMO ... Yes, it was a natural reaction to retaliate in that way, and yes, the officials completely missed the incident that lead to the retaliation at the time, but, a punch is a punch, and if they want to eliminate this stuff from the game, they need to apply a penalty.