I meant that in the US you couldn't really buy anything at wal mart with euros. Im pretty sure that the only currency legally protected to settle debt here is US currency. So at some point you would have to exchange it for us dollars in some way, to be able to use it here. I also know that it was common practice for countries to ban the use of foreign currency, although I am reading that this isnt the case as much anymore due to globalization.
http://en.wikipedia.org/wiki/Foreign_exchange_controlsI don't believe the US bans the use of foreign currency. Along the Mexican border shops routinely accept pesos as payment since the alternative is to turn the customers away. They get a couple extra points on the exchange rate, but that is primarily a tax on the ill-prepared. Similarly (as mentioned above) shops along the Canadian border accept Canadian dollars.
The only reason you can't spend euros at Walmart is logistical, not legal. The cash registers aren't setup to deal in currencies other than the US dollar.
Obligatory link:
http://www.usatoday.com/money/economy/2009-04-05-scrip_N.htm