Passage: The kids visit the Masons and learn that they are behind enemy lines in "occupied America". Robert's father is revealed to have been executed because of the missing inventory from his store. The Masons charge Jed and Matt with taking care of their two granddaughters, Toni and Erica. After killing Soviet soldiers in the woods, the youths begin an armed resistance against the occupation forces, calling themselves "Wolverines", after their high school mascot. The occupation forces initially try reprisal tactics, executing groups of civilians following every Wolverine attack. During one of these mass executions, the fathers of Jed, Matt, and Aardvark are killed. Daryl's father, Mayor Bates, is a collaborator and tries to appease the occupation authorities. Despite the reprisal tactics the occupation forces get nowhere.

Question: Why was his dad killed?
missing inventory from his store