they don't need control in order to put taxes on your trades. they don't have control on all the currencies in the Forex market and they have no control over the stocks in the stocks market but they are taking taxes on your trades and the profit you are making from trading these things.
the same thing can easily be true about bitcoin or altcoin versus fiat trading. but bitcoin versus altcoin trading can be a bit tricky but it can still be taxed in some ways.
i always say when it comes to taxes and the legality of what you are doing it is best not to make assumptions and consult a professional.