They are mandated to exercise their monetary policy powers to pursue specific pre-set goals: the policy objectives for which they are accountable. In the Eurosystem for instance, price stability is the overriding objective. Therefore, although the actions of the central banks may have an impact on their financial risks, these actions are taken with the requirements of monetary (and financial) policy in view.
The Federal Reserve will work closely with our fellow regulators, the Congress, and the Administration to ensure that the law is implemented in a manner that best protects the stability of our financial system and strengthens the U.S. economy. BIS Review 124/2010  3