Ultimately censorship comes from those who have power. The purpose of censorship is to maintain that power by either preventing the spread of 'radical' ideas or to appease another power group which could otherwise threaten the in power group. (i.e. Banning an art exhibit because right wing Christians/Jews/Muslims/Oil Corporations/any other group you can name, stands to be hurt by, or otherwise is opposed to, the content of that art exhibit.)