The Walmart connection makes sense. I think it was in the book by Lt. Gen. Russel Honoré (in Command of U.S. Army Division stateside post-Katrina) that I read Homeland Security/FEMA and Walmart got into bed together after Katrina. The idea was that Walmart is loaded with provisions, medicine, and -most importantly- clean water...and they're everywhere. So a deal was struck that in cases of national emergency, the federal government would take control of Walmarts and use them as supply centers.
It's entirely possible that the military wants to drill on taking over a Walmart for that purpose...and what better way than to actually take control of a few of them. I live within driving distance of Brandon, so I'll be sure to head over and take a look once it all begins.
I also live near a huge privately-owned prison that's on a military installation. (There has been a ton of air traffic around it lately.) If anything were to become an internment camp, it would be that site. Who needs a Walmart with a capacity of a few thousand people when there's a perfectly good prison in the middle of nowhere - already surrounded by military checkpoints - that can hold tens of thousands of people?
If the Walmarts are meant to be anything, it's exactly what already are - storehouses. They're probably figuring out the best way to use them in provisioning an occupying force while defending them and using the parking lot as an MLRS firebase.