There seem to be several factors at play. Firstly, a new towing standard,
SAE J2807, was adopted in the U.S. in 2013 (there was no standard previously and car makers' marketing dept seem to have had the main say in tow ratings). Subaru launched the SJs Forester with the lower 1,500 braked tow rating soon after. Secondly, U.S. speed limits are generally much higher compared to other markets. In Australia the maximum speed on highways is 110 kph/68mp and many have 100 kph/62 mph limits which are rigorously enforced. I understand in much of the EU the maximum speed when towing is the equivalent of 55 mph. Higher and steeper terrain may also be a consideration.
But from a purely power train engineering perspective, Foresters can handle much heavier loads than the U.S. ratings.