[nostrademons]

Governments are social constructions too. They can & do fail - we tend to forget about this in the developed world because our governments haven't failed within our lifetimes (though in much of Europe, only just barely, and in Eastern Europe / Russia / China that's not true at all), but it's a regular occurrence in other parts of the world. When a government fails its power to tax becomes void, and so (usually) does its currency.

The alternative - for someone wishing to transact in an alternative currency and not pay taxes - is to pay foreign or private powers for protection services and then simply not pay the tax. This is suicide in the U.S. because we have a large & well-organized military, but is commonplace in many other parts of the world - and indeed, the U.S. is frequently the foreign power supplying arms and accepting its currency for many of these rebel groups.