At one point of view bitcoin is an instrument for investment so ppl buy it willing to get income in their currency.
At the other point it is a currency and it might be used with a purpose of saving. In this case the tax would be stupid.
I see nothing wrong with taxex as long as the government will use our money for the welfare and common good of our people not to corrupt it. We should counter the other claim of China that they ban the bitcoin because the government cant imposed tax due to crpto currencies is volatile.