The Walmart near the university I attended unionized. Walmart simply closed to the store down, fired everyone, and built a new store near by. That, along with the high turnover, is why employees do not bother to unionize. Blue collar workers may not be 'smart enough or skilled enough' to direct movies about talking piñatas, but they work hard doing shit jobs and deserve to be fairly compensated. They would gladly unionize if they thought it wasn't a futile endeavour.
Anyway, why would Walmart care if they received an image boost? The people who would be impressed would likely never step inside the store even if they were giving away Che Guevara hemp t-shirts. Meanwhile, the people who do shop there would be pissed that they had to pay two extra cents to shop at the now unionized store.