You're absolutely correct. The hit was clean and officials were not reprimanded because it was not deemed to be a penalty.
I can understand if a fan doesn't like the rule, but it was an illegal hit. The rule is pretty clear: "
You can’t hit him in the head or neck, and you can’t use the crown or forehead parts of the helmet to the body.”According to Dean Blandino, the NFL's VP of Officiating, the hit was illegal (which is why the Steeler player was fined $25k) and there should have been a flag thrown on the play (negating the TD).
http://nfl.si.com/2013/12/18/terence-garvin-fined-hit-bengals-punter-kevin-huber/
