I don't think Wal-Mart "plans" to stop selling ammunition. I'm sure it is something that has been considered, but is not the cause of the empty shelves.
I was at Wal-Mart on Saturday night. Even the bulk .22 LR were low. I ask, and was told, just supply issues.
Knowing what little I do, I'm sure the ammunition manufacturers are looking at price increases and:
1) have a low contract price with Wal-Mart
2) would rather fill orders for military contracts
3) are selling to shops where they make more on the order
IMHO, Wal-Mart is slow to adjust to changes in their margins and to a price increase from their suppliers (and in this situation, something that does not happen to retailers like Wal-Mart, they need the supplier more than the supplier needs them).
With all that said, this is only a small part of the possible problem. Demand for ammunition has increased a great deal (and I will not argue if it is for good reason or panic, but it has a lot to do with who is in the White House).