Why do the two men, Tate and Atticus, tacitly agree to say that the stabbing has been an accidental, with Ewell "falling" on the knife?

The sheriff urges Atticus, this once, to accept the situation even if it's not perfect according to law: Mr. Ewell was responsible for Tom's death, and the sheriff urges Atticus to "let the dead bury the dead." He says that it would be a sin to drag shy Boo Radley out into the limelight, and declares officially that Mr. Ewell fell on his own knife.