The notice cited the fact that the currency is not covered by any nation or strong central institution. Prohibition is also part of the fight against illegal activities that are suspected to be paid solely with Bitcoin. Although banks in China may not trade with Bitcoin, it is not forbidden to individuals, but government plans to continue strict monitoring of the financial transactions with digital currencies.
The National Bank also warned citizens that the trade done with this virtual currency is at their own risk. About one-third of the world's Bitcoin transactions happens in Chine, the same value fell after the announcement to about 10 %. The Chinese government with similar move back in 2009 derailed a virtual currency Tencent QQ, so this is nothing new or unexpected.
Any Bitcoin miners out there? Are you worried...