Why did the Hubermanns avoid any discussion of Liesel's mother in front of her daughter?

Liesel's mother Paula gave her up for adoption at the beginning of the novel. Her father had been taken away by the Nazis for being a communist, and later, Paula was taken away as well. The Hubermanns avoid discussion of her mother in order to avoid upsetting her. Liesel eventually understands that her mother's decision was made based upon her own protection of her daughter.