Earlier quoted context omitted.
The US's place in the global economy is just a bit difference than Zimbabwe, so that's a false equivalence. Hint: I was talking about imperialism, not "buy stuff with the national money, citizen!"
The American hegemony is bigger and involves a lot more moving parts, sure, but is that really a difference in kind rather than scale? And if being a global superpower is the key to running an economy based on fiat money, how did it get that big in the first place?
America didn't start with fiat. The transcontinental railroads (http://en.wikipedia.org/wiki/First_Transcontinental_Railroad) provided the backbone for a continental network that helped unleash human creative potential, much like the Internet is doing today. This led to an economic boom. Then two world wars destroyed Europe while leaving America virtually untouched. In the aftermath America had risen to superpower status (http://en.wikipedia.org/wiki/Aftermath_of_World_War_II).