the government has no right to tax that is not provided by their laws. Cryptocurrency has an indefinite status in the state. It is therefore not subject to tax
They have the rights since they are the one who is controlling the society of a country and they can just simply create a law that can shape out and regulate the usage of bitcoin and even put tax with it. And for sure, if the government will see that they have a chance to get some profit from taxing crypto currency, they will.