That is just plain ridiculous. Even my father's 170bhp E39 has double vanos and they didn't put it in the Z3 M, US or EU matters not.
Same with the E30 M3 and Australia, due to to numerous reasons, it wasn't shipped.
The thing is that due to emission they often have to diminish the power of the engine and US has different rules than Europe.
Have you done any modifications to your car, engine wise?