My guess is that criticizing some local supermarket (unknown outside of portland or wherever it was) wasn't as sexy as criticizing big evil Walmart.
Your guess is wrong. From the online book synopsis:
Ehrenreich is eventually hired by both Wal-Mart and Menards (a large-box building supply retailer), passing both the personality and drug tests and enduring their respective new-employee orientations. After discovering that Menards not only back stepped on the initial starting wage of $10 per hour but would demand 11-hour shifts, Ehrenreich opted to accept the Wal-Mart position, despite its lower wage scale.