Rosa Parks refused to sit in the back of the bus, every white person said similiar about her rudeness and lack of manners, and look what happened.  I think she was only tired that day and simply didn't want to move to the back of the bus.  Chelsea was treated very badly and was rightly angry, but the effect was to in one picture represent all the Sanders voters' feelings about having a rigged election and convention.  It may not be for everyone, but neither was sitting in the front of the bus.