Welcome to the world of federal law/regulation. The shower must not pass more than 2.2 gpm at 80 psi (IIRC). To ensure they meet the standard, flow is restricted as you have observed. Many products use a removable washer to limit the flow in the US, but allow for easy removal outside the US. Some US residents even take advantage of this to readily break the law. Other flagrant lawbreakers have been known to drill out these restricted orifices.