How a business runs their business is THEIR business. I'm sure at one point in time the Corporate management of Wall Mart evaluated the pluses and minuses of how they ran their organization, including the use of the property from corner to corner which was the best advantage for their bottom dollar.
So overnighting in a Wall Mart parking lot became a convenience for their customers, not an inalienable "right"! Since then, their generosity at showing good will to their customers somehow became an inalienable "right" and became abused. When the bottom dollar is now affected each local Wall Mart, either because of local ordinance fines or loss of business because of customers avoiding them because of the underlining "fear" that "squatters" are a threat to them (somehow), well, either way ... the bottom dollar affects their profit and they have to retract their stand on over night parking.
It's good to know which Wall Marts allows overnight parking and which do not. But the real problem is not the true traveler that passes through, needs a few hours of sleep, and then travels on. How many folks shop at Wall Mart after midnight anyway? (especially after the store is closed for the night)??? The problem is the abuse of the courtesy and the impact that abuse has on Wall Marts bottom dollar.