From what I can tell, Walmart has been associated with Moosejaw (at least generally) from when they bought it a year and a half ago. It looks like what's changed, and what's inciting chagrin among suppliers, is including a link to a Moosejaw web store on the main Walmart website.
It seems (I guess) like there are two aspects: (i) association with the Walmart brand name and (ii) increased push into broad-market online retailing. The former is really more a matter of image and show, but I'm no expert on branding and marketing. The latter might be the bigger issue. It's an aspect of a wholesale (if you will) change in the retail market for goods: from service-oriented brick-and-mortar stores to big-selection price-driven online retailing, In a sense, it's sort of a disintermediation ... or, anyway, the replacement of the old human and physical intermediaries with a website and a fulfillment system that ships to buyers direct from warehouses.
I don't think that has much to do with Walmart particularly. As noted in some of the reporting, that's really Amazon's doing, and Walmart is just trying to stay alive as people are doing less walking into massive low-service stores to pick off shelves holding a relatively limited selection of quite cheap stuff in favor of sitting at a computer and browsing a much bigger selection of cheap and mid-priced and expensive stuff at what's (in each case) is likely to be at least pretty close to the best price available.