It's an optical device. The frequency is infrared (slightly longer wavelength than the visible light.) The sensor projects a infrared beam on the windshield and measures the reflected light. If the windshield is wet, then the amplitude of the reflection changes, and the sensor is able to detect the rain intensity. A smart control software controls the speed of the windshield blade, and et voilà!
US: MazdaUSA's specs don't mention auto wipers or light control. However, I have seen a pre-production US-spec Mazda3 picture with 'auto' written on the wiper stalk, and Canada is definitely getting them (US gets what Canada gets and more). So we'll have to wait until we actually see a production US-spec vehicle to know for sure.
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