I get where you're going but firstly, this is AFL not Rugby Union. There is stat somewhere about the amount of concussions and head injuries Rugby players have over any other sport. You are way off on the palm versus punch impact ratio, just consider the velocity. Happy to be wrong here but a palm at face versus a standstill punch can do a lot more or equal damage.
Rugby might be the second biggest "played" but is it even watched by enough people? And why football as a measure and not sport. AFL would be in the top 5 football but that's not saying much.
Yes, Aussies have soft heads. That must be it, not the dangers of a reckless way of avoiding a tackle that could lead to serious injuries even if it was dealth to the chest. May as well let players kick to get away.