Why do Heck Tate and Atticus lie about how Bob Ewells dies?

Boo Radley is a mocking symbol in the book and to put him in the limelight would ruin him. People would want to cheer and congradulate him for the death of Bob, but Boo is too shy and timid to be placed as center of attention. They lie because it would be a punishment for Boo to tell that he killed Bob after saving Jem and Scout.