There is no nation-wide trend to stop selling firearms at Wal-Mart. The one's here in SE Wisconsin that I frequent all have a firearms counter. The stock changes with the seasons it seems. Leaning more towards .22 plinkers and general utility shotguns in Spring, and more towards waterfowl shotguns, and centerfire deer rifles in the fall. They've also expanded firearms accessories, and are carrying more air rifles and Airsoft lately.
It's pure local economics/demographics issue. A location will discontinue firearms for poor sales, state and local regs on firearms, or the potential for bad publicity from firearm sales to a low income/high crime area.
Any of these factors make it not worth the hassle to sell firearms. Whether you love them, are ambivalent, or hate them, everyone should recognize that, for better or worse, Wal-Mart is a hyper-efficient company that will not knowingly waste a dime. They do it well, and it's why they dominate.
Just like water flows naturally to the lowest point, and follows the path of least resistance to get there, the products in Wal-Mart's supply chain flow to the stores where they sell the best (or their analysts at least think they do), where they sell the easiest, or where they have other benefits; like firearms driving ancillary sales of ammunition, accessories, or other sporting goods..
The Wal-Marts that have opened in China have tanks of live turtles.
In the pet department?... Nope.
In the "fresh food" aisle of the grocery side. It's called "addressing the needs of your customers".
If they sell firearms in an undesirable location, it's wasted space, time, and profit where they could be selling something else.
Would you start a surfboard company in the middle of Nebraska? Would you put up your own money to open up a gun store in Berkley or Compton CA? How about a tanning parlor in the middle of Death Valley? Perhaps you'd like to invest in a Hummer dealership in the middle of Manhattan? Better yet, what about a mountain climbing store in Florida!...
Whether a Wal-Mart sells guns or not says more about your local politics and demographics than it does about Wal-Mart. I agree that seeing your local Wal-Mart discontinue firearms sales is a concern, I would be worried if mine did. But all it means is that your area has somehow become less firearm friendly, not that Wal-Mart has.
They are not the NRA, they are a business.