I don't see Walmart closing a store for Jade Helm, but I do see the idea of the Jade Helm planners working with Walmart to temporarily use one of the closed down stores as a defacto FOB for certain elements of an exercise. National Guard armories are only so big, but traditionally, National Guard armories and shut down schools are used for those kinds of training exercises. There are also a number of mothballed military bases around California in particular which can be used for that purpose, but quite often the facilities there don't have the kind of easy access trucking depot setup that are relatively common at Walmart, or they still want to keep the layout secret enough that contract truck drivers don't get to see everything and put it on social media video once they have been in there.

As it is, by using Walmart, if some guy starts uploading video of a truck being unloaded at a walmart loading dock, nobody can even conclusively tell where it it is and may not even know what's in the boxes, so if he goes off on some conspiracy theory sounding stuff, its not like it would be readily believable if the military people are there in civilian clothes.

If I am not mistaken, the way this is supposed to work is that the military can move in and conduct operations without the general public even figuring out who they are or what they are doing.