The move is a response to a new GST policy that will apply 10% tax to all overseas purchases under $1,000 announced by the Turnbull government last year in a bid to “level the playing field” between Australian and overseas retailers.
Already happened with Valve. Buying from the store has 10% in the price, but not all prices changed.
There may also be some organizational lines being crossed; I can imagine the American site team doesn't really want to deal with Australian tax issues.