I am betting that the premium "requirement" is buried deep within the owners manual. The published engine numbers are attained using a higher octane fuel and if the owner wants to access the published power then he has to pay for it. Regular fuel CAN be used but the engine will not make the published numbers.
Premium is probably reccomended but not required. If it were my car I would man up and pay for the premium. The engine can retard the timing to compensate for the lower grade of fuel but just because you can doesn't mean you should.