Walmart is so big it's in charge. They dictate the price they'll pay to a manufacturer for the goods they make. In order for the goods to be placed on the shelves at Walmarts around the country, the manufacturers have to accept that price. If they don't, then they lose the possibility of being sold at Walmart.
Walmart's charitable giving is small compared to what it should be considering the business they do. It's just big enough to be more than that of smaller chains and stores.
I used to buy non-food items at Walmart rather than the grocery store to save money until I realized that I wasn't saving enough to make it worth my time. I have a big Walmart less than a mile from me, but I don't set foot in the door as often as monthly. I hate to see them run smaller stores out of business. I try to buy locally.