You're misunderstanding the constitution and what the "right to create currency" means. There are many currencies in circulation, and there have been many state and even private currencies over the years. Even today there are localities that make their own currencies- such as the Ithica Hour http://www.ithacahours.com/
What the constitution reserves to the federal government is merely the power to determine how many grains of silver are contained in a given dollar.
Thus, actually, the current paper US dollar, which is not denominated in silver, is illegal, it is counterfeit!
Thus it is clear that tradition has long ago seperated from the law. But this is relatively recent. In 1930s it was that the government made ownership of gold illegal and switched from silver certificates to unbacked dollars, but even then the dollar was backed internationally with gold, until Nixon closed the Gold window in the 1970s and defaulted the US government.
An excellent book on the history of Money in the USA, is "The Creature from Jekyll Island" by G. Edward Griffin.