Why does the narrator get so angry when he notices the cast-iron bank in the shape of a black man in Invisible Man

Standing by the pipe, he notices a figure of a Negro with overly exaggerated features which he determines is a bank. The bank's structure is such that the hand flips the coins into the smiling mouth. The degrading Negro image must be fed with money to be kept happy. The "self-mocking image" drives the narrator crazy and he uses it to smash the pipes with.