Walmart has stockholders that have to be considered. Whereas a privately owned company can make any decision they choose, good or bad, a Walmart can't just pay people more money because they feel they should. If they do that, it cuts their profitability and suddenly Target is neck and neck with them (performance-wise) and become more attractive to investors. Walmart is not going to jeopardize that.............ever. They can't.
Beyond that, pay isn't supposed to be based off your family situation. If all Walmart employees have 3 kids, no education, basic skills, average work ethic, etc... why is there any expectation that they should "get" something to equalize them with people who have an education, valuable skills, good work ethic?
Let's not forget that if Walmart makes more profit, they also pay more taxes, it's not just a one-sided equation where Walmart's practices only cause tax burden, they also cause tax revenue.
It boils down to what Meister already said, you can't bitch about Walmart if you're shopping there. You have a choice, and if so many people believe the "what's it going to hurt to pay a little more" theory, then clearly they aren't practicing their own beliefs because Walmart can't be that successful if everybody that claims they don't go there, doesn't go there.