Why can't the Walls kids kill the flies that are constantly buzzing around their house?

The Glass Castle was published in 2005 and is a memoir by author and journalist Jeannette Walls. The book details Walls's unconventional and nomadic upbringing as her family travels from state to state, their circumstances worsening with each move. Walls's parents can be considered dysfunctional; her father is an alcoholic unable to hold a job, and her mother is an artist who often leaves the children to fend for themselves. The title of the book is drawn from "the glass castle" that Walls's father promises to build for the family; it symbolizes illusions and her father's inability to focus on the present reality.
Walls's mother is equally distant from reality, often believing that her children are best off learning to become self-sufficient. It is this reasoning that leads her to forbid the children from killing the flies around their house. While living in Battle Mountain, Nevada, Jeannette realizes that her friend Carla does not have problems with flies in her house—her family uses pest strips to trap them. When Jeannette brings this up to her mother, she is told that the flies are part of the food chain and that killing them will disrupt the order and the other animals' ability to be self-sufficient. The flies are food for the birds and lizards, who are food for the family's cats. If the flies were killed, the cats' prey would die, leaving the cats unable to fend for themselves.