It's importantly unlike what the OP suggests because Walmart has insurance and, as far as I know, has not received any special legal immunity, much less total, unqualified immunity. It owes the same responsibilities to "invitee" as any other person or entity does under applicable state law.

I have actually read a couple of articles about "Walmart camping." According to employees of the company, it does generate sales. A lot of sales at some stores. Most of those who take advantage of it are RV drivers. In some places where it is frequent, the stores place some RV-related products (e.g., septic system treatment products) near store entrances. All that is typically asked is that RVs be parked in designated areas, away from the parking spots that are most convenient for day shoppers. I would be surprised if one would be allowed to pitch a tent directly on the parking lot surface. Seems like a recipe for an accident.