As has been said, the price of a return trip from point A to B is rarely the same, or even nearly the same, as the price of a return trip from point B to A. The airlines will charge what the market will bear. Presumably, there are more people originating in Sydney than there are in Paris on the particular route you've highlighted.

And besides, the majority of passengers from France, or Europe, would not even consider flying to Australia via the US or with an American carrier, instead choosing to fly via Asia, so perhaps UA is trying to attract customers by offering a lower price. However, €1,500 isn't even the cheapest fare to be had from France.