why does heck Tate insist that bob Ewell fell on his own knife

The struggle between Jem and Bob Ewell is blamed for causing Bob to fall on his knife and die. Heck Tate chooses to keep Boo out of the proceedings as he sees Bob's death as natural justice. Also, Boo had been punished enough by being incarcerated for most of his life, and he had acted to defend Jem from Bob Ewell.