That's how a win-win works. If Wal-Mart can make a profit by getting people to save money there's nothing wrong with that. Especially when compared to the banks that make significant money fining you for being broke.
Except that by giving walmart money that could be invested conservatively, you've actually losing money to inflation. True, this is exactly what most people do with their banks, but banks aren't win-win either, they're both businesses that offer these services because it makes them money.
How do you expect the factory worker to start investing her <$500 savings?
Interest is a waste of time anyway these days. My credit union is paying 0.10% on savings accounts under $100k, and something like 0.25% over that amount. A "high yield" online only account might pay as high as 1%.
Often the poor are poor because of a lack of financial literacy. I've seen it in those around me, and in my own parents. Getting people to save money for a rainy day is powerful. I've started doing microloans to help lift my friends up too, I made it.. and with a little help, maybe I can lift them up with me.
The people who use the WalMart debit cards are not the kind of people who invest money, or even normally save it. They are poor people who often have zero cash savings, and now they have some.
Your first $1000 should be considered an emergency fund much more than an investment. Everyone should have or work towards having $1000 to their name, but way too many are living paycheck to paycheck and saving literally nothing.