Actually this is legally valid. The same occurs in the US. Any currency that is defaced or torn is deemed invalid.
""Banknotes on which there are writings or are stamped or have any additional signs will be invalid," Iran's Jam-e Jam newspaper quoted Mr Darvishi as saying on Thursday."
you're right sir but please read the whole entry this seems against the free speech..the slogans even condemning dictator(not by wishing death to him) will be considered invalid as well..however i fail to have an opinion on this since iranians are smart and have a media access plus internet to vent out.