There is no defence for Luke Ricketson IMO. As some have covered, the only man on the field who may have had some argument to "retaliate" is captain Brad Fittler.
I do not question Luke Ricketson's loyalty to his captain or his club, I do not question the fact that Luke Ricketson is "a bloke you want to go to war with" either. I do question the need for Ricketson to hit the unexpecting Nathan Fien who looked as if he was going to stop the near altercation between his hooker and Fittler.
The incident does not come near that of Danny Williams and Mark O'Neil. Williams made a bee-line for a player who was again unexpecting, but playing the game, his mind on the job so to speak...from memory he (O'Neil) was returning to the defensive line. Fien was amongst a possible altercation.
It does not excuse Luke Ricketson IMO. He should miss the GF at least for taking the 'law into his own hands' which was not needed. This action was what sparked what was too follow.
I have said before, I do not try and understand the NRL judiciary. The lack of consistency is embarassing to say the least (Ben Ross grading in comparison to recent 'dangerous throw' charges for example) and it is becoming a bit of a lottery when trying to predict the result.
I will end the post by saying Luke Ricketson does not deserve 18 weeks on the sideline for the incident. IMO what she he recieve ? I can't give you an honest answer (refer to the above paragraph) but will say Luke should not be playing in the GF.