If I use actual coins or paper money, you might be able to find out how much I have, but you can never tell what I've used it for. Your supposedly private "coins" can be traced back to the moment of their creation if someone can connect you to the wallet. That means there's a complete, unfalsifiable, unarguable accounting of everything you've done that can be used to hang you. All anyone needs to do is show that you had access to a wallet from which illegal payments were made and you will be convicted.