There is a very old and long thread
on this site about walmart parking. But the net is that Walmart allows RV'er to overnight park. As a result folks will shop at walmart for food, rv supplies, and anything else they might need.
Many rv'ers will spend the night due to the cost (none), convenience (close to interstates, late arrival, early departure).
There are "guidelines" for staying at walmarts such as no jacks on hot asphalt, don't setup camp, park out of the way, etc.
I've never parked in a parking lot, but have no objections to doing so.