American copyright law has always performed a complicated balancing act involving both commerce and culture. It tries to protect products of creativity so that people have economic incentive to keep on creating, so that a new movie, for example, is not immediately copied and resold on Canal Street, depriving the moviemaker of the possibility of income. But the law has also evolved ways to allow for creative uses of copying: the fair use exemption, which allows some copying for things like criticism, comment or news reporting.
Richard Prince Lawsuit Focuses on Limits of Appropriation – NYTimes.com.