Ok - here's the official answer: retaining the existing towing capability required extra cooling capacity, which added significant cost to the vehicle. After considering the towing capacity of competitors (typically 1500lbs) we developed the US version to tow to that level. The Outback has greater cooling capacity and as a result, the towing weight limit is greater.
In Australia, towing is much more common with this class of vehicle. As such, FHI equipped Foresters for that market with the additional cooling capacity, giving them a higher trailer weight limit. HTH.