My local Wal-Mart doesn't have M1 15W50 (qts or jugs) and says they won't stock it any more. They also didn't have any Truck & SUV 5W40 jugs nor did they have shelf space for those jugs. Are you guys finding the same?
I've observed that all WalMarts do not all carry the same motor oil and such. Some will have different product mixes from particular dedicated vendors, which also seems to extend to motor oils on their shelves. For example, one new SuperCenter only has the 5W30 and 10W30 oil, with a small amount of 20W50 whereas another local one will have 10W40 in their product mixes. One carries Castrol Dexron III and the other one carries it in Pennzoil and Tech brands. Probably has to do with the particular market area and "what sells" there.
Just because one product might be removed from the shelves or closed-out would not necessarily mean it was being discontinued by the manufacturer, just that its sales were lower than anticipated. They have to keep their shelves "productive".