Why are blacks so offended by the fried chicken stereotype?

Because all stereotypes are harmful as they encourage a certain amount of dehumanisation. There's no such thing as a 'positive stereotype', simply because by lumping people into arbitrary groups you seperate them from yourself. It all fuels the harmful "them and us" mentality that is, basically, the root of most of humanity's problems. Furthermore, by judging people on an (often unfounded) clich� you are completely failing to look past their outward appearance and accept their own, unique character.