Alas, this is not unusual. Although Walmart corporate's policy is to allow overnighting, each store manager gets to set the policy for their own store. In some Walmarts, the parking lot is actually owned by someone else, so the manager can't legally give permission--in other Walmarts, local ordinances make it illegal no matter what the Walmart policy is. And some managers ban parking in their lot when some dumbfuck does something stupid like dump their trash in the lawn or empty their pee bottle on the sidewalk or move in permanently and live like they own the place.
It's always a tiny handful of shitheads who end up wrecking things for everyone else.