The $100 price is a relic of when the Australian dollar sat between 50-70c US. It has absolutely no reason to exist today when selling digital products. The only reason it exists is publishers say "they've paid it before, so they can keep paying it" even though their profit on that $100 would have increased dramatically because instead of getting around $60-70US, they're now getting $100US because of the exchange rate.
And we're talking about digital products here. It costs EA (or Take Two, or which ever other knob head publisher decides price gouge) no extra to sell a game in Australia than it does in the US via digital distribution. It's a global market place. The $100 price is a relic of the past and it's not at all unreasonable for people to be angry that publishers are trying to cling to it in the digital age.