If Walmart engages in wage theft and is then sued for it, that sounds to me like the law works. Unless the penalties aren't enough to resolve the issue or something, that doesn't sound like an ongoing injustice - and at any rate, sounds like a problem for the courts, not the public. Sorting that sort of thing out fairly is their job, and they're surely going to be more thoughtful about it than angry internet mobs.
If Walmart employees can't support themselves and wind up taking advantage of charity - either privately or through the state - isn't this also the system working as intended? Systems are in place to help low income individuals. These are low income individuals. It strikes me as strange moral reasoning to say the low market price of unskilled labor is Walmart's fault.
It strikes me as particularly strange moral reasoning to charge the use of welfare in this case to Walmart's account and call it a subsidy. That would only make sense to me if the people earning that wage would do better if Walmart wasn't there. I don't think that's the case, though. Unskilled labor doesn't pay that well whether you're flipping burgers, delivering pizzas, stocking shelves, or waiting tables. It's not the fault of the companies offering those jobs - it's just what that sort of labor is worth.
But Walmart should just pay them more, everyone deserves a living wage? I don't think I agree. I think there is room in the economy for second incomes, for student jobs, for seasonal work. Not everyone is a single head of household, and if you don't have the skills to be one, often you just - don't move out. We have no shortage of help - private charities that do vocational training, societal safety nets to keep things from getting too bad, most people get by with help from friends and family until they can make it on their own. I think there's nothing wrong with that.
One of the things I would be concerned with is insisting that Walmart pay its employees more. That will surely come in the form of raising prices, and the very low prices at Walmart are a boon to low income folks - both those who work at Walmart and those who don't. I am skeptical that making Walmart less efficient will help the communities it's in, compared with the social safety nets and private charity - generally provided by the better off. Forcing Walmart to pay higher wages strikes me as taking from the poor to give to the poor.
On the contrary - the folks I know who work at Walmart are content with the situation and doing ok. They know they could go get a vocational degree or something and make better money as a welder or something. They don't want to. It's an odd job between stages of life, not a career, and good enough for the moment. If there are enough people like this to run a whole business that hugely benefits the poor by offering goods at low prices, isn't that system on the whole a good thing?