It says even the 1.6i can tow 1500 kg.
It's all marketing, just another imaginary constraint to shove North American buyers into a more expensive car. This is the same reason we can't get leather in anything but the Limited, yet ROW buyers can get leather all the way down to the 1.6.
It's very simple. If Australians can't use an Impreza to tow small motorcycles or watercraft, they will buy a competing vehicle that can, like the Corolla.
For a company that prides itself on all things outdoors, this is unimpressive.