Why can't a person leave the Jehovah's Witnesses quietly and peacefully?

If a person is guilty of immorality or dishonesty, or joins another religion, one could understand people being warned of the possibility of that person trying to involve others in their misconduct. But, is a person simply decides "This is not for me," why do elders treat the person like a leper and instruct everyone in the congregation to treat them that way also?