It's not fiat, it's sound money. And people think twice before wasting sound money.

Those examples show that in the absence of effective legal tender laws, Gresham's law works in reverse. If given the choice of what money to accept, people will accept the money they believe to be of highest long-term value, and not accept what they believe to be of low long-term value. If not given the choice and required to accept all money, good and bad, they will tend to keep the money of greater perceived value in their own possession and pass the bad money to others.