As a libertarian, I definitely don't want the government able to track my transactions as I'm doing them. But as a citizen, I'm obligated to pay my taxes. If my cryptocurrency balances are accessed via a smart contract I agree to when I sign up[ on a cryptocurrency exchange to enable my account/encrypted key to be charged an automatic fee by the country where the IP address is located for the capital gain according to the tax code of the country upon a sale/withdrawal of the currency - this capital gains tax calculated as the difference in fiat value as of the date of purchase and the date of sale and the current tax rate on capital gains, my taxes are paid and my transactions remain anonymous.
Every country is entitled to tax its citizens. If I'm a criminal (money launderer), then it makes sense for law enforcement within my country to discover that and prosecute me. That is a separate issue from tax evasion. Conflating the tax evasion with other kinds of crimes is misguided. If South Korea wants to prevent people from trading, that is simply reflection of the desire for over-control of citizens by the existing political establishment. They can do it, just as any anal country can regulate free enterprise within its borders - stupid thing to do - but politicians can be voted out and idiots removed from office. That is not an inherent problem for cryptocurrencies, but for political jurisdictions. 