What law in what country says that someone wholly or partly has to use bitcoin?
It's obvious you misunderstand the full definition of "fiat". Fiat can be a government issued currency but it also is defined as a currency with no intrinsic value. Bitcoin, like USD, only has value because we agree that it does. Thus Bitcoin is simply another form of fiat currency. Bitcoin though, is not debt based. So thats good.
With word relativism, anything is possible.