I'm only guessing, but I would Imagine this generalization has its roots(no pun intended) in the days of early US slavery where the slaves were allowed to keep/raised chickens in their shacks, and naturally they ate them due to not getting much else good to eat.
And seeing as how they didn't have a proper kitchen to cook them in the usual manner of the day, they fried them over a fire in leftover grease..
And Mamma passed the fried chicken recipe down to their daughters and they served it to their kids even after they had other things to eat due to chicken being a relatively cheap meat.
Sounds good don't it ?