I would argue that Walmart pays minimum wages because they can and their customer base doesn't really value the benefits of paying higher wages and 'participatory management' - primarily lower turnover and thus more engaged/invested employees who deliver better service and higher productivity. Walmart most likely sees their employees as replaceable and the average Walmart customer is driven by low prices and not great customer service. To that end, the lower the Walmart cost structure, the more margin they have to work with to stay competitive (without having to lower CEO salaries, etc. of course).
Costco and Trader Joe's are known for great customer service and easy return policies. Based on my observations, people who shop at Costco like to save money, but it's from a best value angle and not from an absolute cheapest I can get angle. The Costco consumer has to be willing to shell out an annual membership fee up front and you're buying in bulk which means you have to be able to pay for goods you use in the future. Trader Joe's wants to be the neighborhood store and that means they need happy employees, not employees who are thinking about how they're going to pay for the knee surgery they badly need.
I'm probably making broad generalizations of the different customer bases here but it's what I've personally observed.