China's concern is capital flight. They dont want bitcoin to be used as a means to smuggle capital abroad without going through capital controls.
That is why they banned financial institutions from funding bitcoin purchases or converting bitcoins to cash, but allowed individuals to use bitcoin as a currency for trade.
Isn't through banks the best way to control capital flaw? I don't get what you're saying.
If you can't buy them with their money then you can't send that money out of the country.