Fuel & emissions really aren't an issue. The ECU can be tuned to run with our grade gas and with a proper tune and equipment (cats, charcoal canisters, etc.) anything can pass emissions. I think in the U.S. it has more to do with the "torque" factor and the "displacement" factor of the average American. Subaru knew they could sell more cars by targeting the car toward the American market. The 3 cats on the U.S. WRX are strictly there to make the WRX qualify for the LEV emission standards, this helps Subaru in the grand CAFE standards game. For 2004 Subaru actually considered removing the cat before the turbo to slightly boost the performance for the production cars, but it increased the WRX's out put to close to 250 HP which Subaru thought was so close to the STI's output it might compromise sales of the STi.

Reversely, in Japan the car registration fees are based on the displacement and power of the motor. I doubt many would be willing to pay the extra fees associated with the .5 liters of displacement and 20-25 HP.