In lieu of an official policy declaration, I wouldn't draw any conclusions based on an anecdotal observation in one store.
Self-checkout machines are designed to speed the flow, and anyone who's ever used one knows that they can be anything but fast when paying with cash, especially when coins are used. And while some cashiers may be slow, they can still end up faster than people who struggle to use the machines, or are just plain slow at checking items and bagging them.
Sometimes it's just faster to check out with a human. And that doesn't count the times when the machines may be broken, or partially broken, like with the change dispenser, which requires the pit boss to come over and reset or reboot them, which can take many minutes. The fact that the pit boss' command terminal has a cash drawer tells you much potential there is for things to go wrong when they're asked to deal with cash transactions. More than once, I've had them issue change through it. It probably makes more sense to just limit those machines to electronic transactions.
But, Walmart has every reason to favor cash over traditional credit payments because they cost the company millions of dollars in transaction fees.
To do that, it went so far as to band together with other retailers (like CVS) in MCX to develop CurrentC, an alternative payment system that would bypass the traditional Visa/MC/AMEX/Discover card cartel and their fees.
CurrentC was a poorly conceived system, and deservedly went down in flames before even getting off the ground, but Walmart hasn't given up.
It's shunning Apple/Android/Samsung Pay in favor of its own NFC payment system. WP will take all major forms of electronic funding, including traditional credit cards; it doesn't link to checking accounts like CC did. It won't help it avoid those dreaded fees, but allows it establish a competitor in the NFC payment game, and capture customer data, which has tremendous value in today's world.
NFC payments may have a future, but neither cash nor credit are going anywhere any time soon.