I think the same happened to Linden Dollars (Second Life). However, I don't see a problem, as presumably they can only tax you on the corresponding dollar value

I think the same happened to Linden Dollars (Second Life). However, I don't see a problem, as presumably they can only tax you on the corresponding dollar value of the currency. Like if you earn 1000 BTC and they are worth 1000$ at current rates, you are taxed for 1000$, if they are worth 0$, you are taxed for 0$.