Tom Robinson cannot accuse the Ewells of lying, because "blacks can't accuse whites of lying."
In a Nutshell, the character Tom Robinson cannot accuse Mayella Ewell of lying due to the fact that he's a black man and she's a white woman in the Jim Crow South. After the Civil War, whites made and introduced laws aiming to rob the newly-freed blacks of their rights and keep them subservient.
