It was also in the best interests of sportsmanship to all competitors not to give favor to his son. Unser Sr. also knew his days were numbered as a competitive driver on the circuit, while he knew Unser Jr. had many years ahead to have another chance at the title (Al Unser Jr. would indeed win the championship twice - 1990 and 1994).
He held on to finish fourth, and thus won the championship by 1 point over his son. Unser Sr. afterwards expressed some regret about snatching the championship title from his son, but felt it was his responsibility to his own team and his own sponsors to race to his ability all the way to the end.