Why would Hinton not sign the paper the police gave him when they brought him to the station after his arrest?

When Hinton arrived at the police station, he was taken into a room where three police officers, Vassar, Miller, and Acker, were waiting with David Barber, the district attorney of Birmingham. They handed him a blank sheet of paper and told him to sign it. He refused because he knew better than to sign a blank piece of paper.