I think you are mixing things together. It's not everything that is legal tender can be task. Are stock legal tender? No, are businesses legal tender? No but I understand your point as you are coming from currency angle but tbe government has the right to tax anything that gives you profit as a result of investment whether it's a realize investment or not realize, you will pay that tax when you make enough profits from your investment and every country has thier rules regarding such.

In my country, the government don't like Bitcoin and don't like associating with crypto but they take taxes from the exchanges from providing services to their citizens. As long as they have find many alternatives to stop people from crypto and it's not working, they regulate the exchanges and taxes them, Like Kucoin for example now charges it's customers to more fees from trading fees and withdrawal fees as value added taxes, this is why I said every country has their own model of taxes in crypto and it doesn't have to be a legal tender before they ask for tax.