Its a formal warning issued by central bank of any country which controls and regulates all other banks in that country.Whenever a new project or scheme comes related to economics,central banks issues a warning to people about it.Even the reserve bank of india issued a warning that bitcoin is very volatile and risky investment.But later,indian government changed its stand about bitcoin and had decided to legalize bitcoin.So,that's no need to worry about.But normal banks considering bitcoin as a rival is another issue.