Why does Mr. Tate insist that Bob Ewell feel on his own knife?

Because he wants to protect Boo Radley. It's obvious to him what went on here and that Bob Ewell is in the wrong and Boo Radley shouldn't be put through a public courtroom experience for doing something good.

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.