IE - government can force Visa and Mastercard to shut down all bitcoin exchanges whenever they want to, effectively killing the currency for all intents and purposes. They only reason they don't do this is because it is not relevant enough to care.
Bitcoin exchanges don't accept Visa or Mastercard. Paypal specifically disallows its use for Bitcoin.
Truth has always been found to promote the best interests of mankind... - Percy Bysshe Shelley