What's the meaning of this quote from To Kill a Mockingbird?"As you grow older, you'll see white men cheat black men every day of your life, but...

These words of Atticus Finch are said after the trial of
Tom Robinson in response to Jem's questioning of the verdict against
Tom:

"You just can't convict a man on evidence
like that--you just can't"

Atticus explains that the men of
Maycomb can and did exactly that because despite what the U.S. Constitution demands, Tom
Robinson did not have a "square deal" in the courtroom. For, the jury members carried
"their resentments right into a jury box." This egregious act of the jury is what
prompts Atticus's words that no matter what family a white man comes from, no matter
what accomplishments a man has, if he cheats a black man of his constitutional rights,
that man is "trash"; that is, he is unconscionable and lacks any dignity and character.
Further, he tells the children that he is sickened by "low-grade white men" who take
advantage of a Negro's ignorance.