The persistence of the attacks is particularly significant because the memorial itself has now become part of the story of how America remembers—and sometimes resists remembering—racial violence.
The memorial itself has become a symbol.
There's an interesting feedback loop: once the sign became famous for being vandalized, damaging it could itself become a form of provocation or publicity. Historian Dave Tell has argued that the vandalism has become part of the site's history rather than something that can simply be erased. Source
The Emmett Till Interpretive Center has planned additional security around the barn where Till was murdered, including cameras and lighting, partly because of the history of vandalism. Source
The replacement marker was shot more than 100 times over several years. Source