Why would the Chinese do that? When the Chinese are buying bitcoin, they withdraw capital from the country, but when they mine coins and sell them for hard currency in the amounts which now provide Chinese miners is to attract investment in the country. Do you think that the Chinese government is so stupid to give up the income?
And let's all think about that a little more clearly: legal Bitcoin in China is actually a drag on the yuan denominated BTC price. Miners sell into the market in large quantities, and presumably take the majority of their profits in yuan. Banning Bitcoin will cause the yuan to crash against BTC over the long term, and banning mining will simply make the crash worse.
It's like this when a government "bans" any good, depressing the supply is easier than depressing demand. And eliminating either supply or demand is not possible.