For any income-generating activities, the government is entitled to tax it. Therefore, such a requirement is completely legal. As for the question of whether cryptocurrency owners should pay such a tax if they can hide the amount of income from the government, it should be borne in mind that public authorities are still able to establish the total amount of income to be paid, and if citizens refuse to pay it, this could be the reason to limit circulation or even prohibit cryptocurrency in a particular state.