More and more countries, like Chile or India, are trying to tax their citizen for the crypto they hold.
Please don't pay taxes on your coins.
You really don't need it, unless you are totally ignorant.
It's simple: they cannot prove you have the coins on your PC/phone , so you don't have nothing to pay taxes on, as far as they know.
It's just their attempt to trick the ignorants that don't know about that 
Even if they look for your wallet on your PC/phone ,
your wallet data is not accessible to them because it's encrypted with password ( uhm.... have you set the password protection on your wallet... right?

).
Of course you are not going to give them the wallet password.
Oh.. they tell you that you withdrawn the coins from an exchange, so that's the proof you have the coins?
Just say that it was an hacker that stole your coins from the exchange, not you.
You got robbed, and you don't have to pay taxes on the money that was stolen from you, right?
P.S. for the geeks: use Veracrypt

I partially agree with what you say. Countries cannot demand tax from anybody without a legal arrangement. After that, taxation will not be easy. Because cryptocurrencies are in a structure that cannot be traced. So when you determine how much money you have is not easy. Especially when the global crypto money exchanges are active.