By: Dr. Spock
But I think the idea is that in the 1920s they hired a bunch of white guys (and not black guys) to play goofy over the top stereo-type black guys who act and talk like retards. People are offended because, while the characters are clearly retards, they weren't even paying black actors to do it.
The Amos 'n Andy TV show in the early 1950s changed that.