what reason does atticus give jem for tom receiving such a harsh sentence?

Atticus indirectly cites racism as the reason for the harsh sentence Tom receives.

“Atticus—” said Jem bleakly.
He turned in the doorway. “What, son?”
“How could they do it, how could they?”
“I don’t know, but they did it. They’ve done it before and they did it tonight and they’ll do it again and when they do it—seems that only children weep. Good night.”