How is that different from any other country? Its a generic law that every other country has, you can't start spreading a new currency in United States as well because "official currency of United States is US Dollar".
We all know what happened to a "Liberty Dollar" in United States [1]:
In 2006 the U.S. Mint issued a press release stating that prosecutors at the Justice Department had determined that using Liberty Dollars as circulating money is a federal crime. The press release also stated that the "Liberty Dollars" are meant to compete with the circulating coinage (currency) of the United States and such competition consequently is a criminal act.[2]
[1]: http://en.wikipedia.org/wiki/Liberty_Dollar#Federal_Governme...
[2]: http://www.usmint.gov/pressroom/index.cfm?flash=yes&action=p...