Cunninghams and Tom Robinson

There are some people in Maycomb that are good people, that they believe that the Cunninghams  are good people and that Tom Robinson is not guilty, but other than Jem, Scout, and Atticus, they don't believe in both. In third or fourth chapter of the book Scout and Jem tell Will Cunningham that he could come over for dinner. Once they brought him to their home, Aunt Alexandria didn't want Will there. But them Tom Robinson's case came up and she believes that he is just a regular person we should treat him the same. So why are the Cuunninghams any different?