This is precisely the sort of misinformation that gets brandished about, but betrays a complete lack of knowledge about how bitcoin actually works. In order to fork bitcoin you'd need to acquire 51% of the network's hashing power, this would require a multi-billion dollar investment in the required hardware to pull off.
I'm not saying a company like Walmart couldn't do this but it would make absolutely no economic sense as it would effectively destroy the value in the network as the trust element has been subverted. The reason bitcoin can act as a store of value is because it is built upon a system of distributed trust. So Walmart would be better off using their hashing power to mine bitcoins as this would generate a greater return. Sure Walmart or any opposing entity with the financial will could commit an act of sabotage to destroy bitcoin, but it would make absolutely no sense.
And yes Walmart could even setup their own crypto-currency, but that wouldn't be bitcoin it would be WalmartCoin and it could never be used to dilute the value of Bitcoin. And unless WalmartCoin can add any significant utility over Bitcoin it will likely just languish, like the 100s of other alternative digital currencies out there which when combined are worth a fraction of bitcoin.
The intrinsic value in bitcoin is the network that secures the trust. Also show me any other payment system that can be used to move millions of dollars of value around the globe, requiring no bank account and costing just a few cents.