Subsidies violate the idea of the free market by artifically supporting inefficient industries that wouldn't be competitive otherwise. Tariffs are economically inefficient in an atmosphere of competitive trade, but may be warranted given unfair trade practices on the part of another country. In the case of China, for example, the Chinese control the float of their currency, artificially keeping its exchage rate low, and thereby, keeping the relative prices of their goods low in comparison to the U.S. dollar. Tariffs would be warranted in the case of China's unfair trade practices in order to compensate for the artificially low prices of Chinese goods.