Since when has what the Americans pay for anything been the same as what everyone else pays for the same thing, especially for games in Australia and New Zealand, where for a retail game we pay so much more. Our pricing system is not based on what the price is in the USA.
The reason we pay higher digital distribution version prices for FULL GAMES, which are also released at retail, is so they're priced closer to the retail prices set by the local distributors.
However, as Hearthfire isn't available at retail, this doesn't hold water. I don't see any reason for this regional price fixing of online-only DLC over what is effectively an international medium (Steam)