Why does Rev. Sykes and the other Afro-American spectators stand for Atticus, even after Tom was found guilty.

They stood as Atticus passed as a sign of complete respect.

"I looked around. They were standing. All around us and in the balcony on the opposite wall, the Negroes were getting to their feet. Reverend Sykes’s voice was as distant as Judge Taylor’s:

“Miss Jean Louise, stand up. Your father’s passin’.”

This is a sign of respect and admiration. Atticus did his best and hardest to defend Tom Robinson.