Just watched the penalty incident again.... Fox's arm goes up as an instinctual balancing motion at the same moment that he lunges into the tackle with his leg. Being the sad individual that I am I then did a picture search on Google for similar tackles and, almost without fail, the defender raises his arm as part of the lunge motion.
Some you win, some you lose, but I don;t think Fox did anything that the majority of defenders wouldn't have down in the same scenario.