What could happen is CryptoRuble will be used as a step to legalize and regulate cryptocurrencies and exchanges in Russia. Russia has been very progressive in banning foreign websites. Even localbitcoins is banned. So what they can do is ban all the exchanges, that do not use cryptoruble, and Russians will be able to buy crypto legally with cryptoruble only. That explains a 13% tax. This certainly is not how crypto was designed to work - in unregulated, decentralized way. I think that is what China is also preparing now for its citizens. Lets see how will it work out.