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.
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.
This isn't money that could be invested conservatively - this is money that might be needed tomorrow, or next week, or next month.