There is a correlation between the income levels of the patrons and the amount of dripped oil you see in the parking lot of any store.
Walmart probably has middle class shoppers more toward the 50K family income level than the 100K level. These folks drive what they can afford and if the old ride leaks a little oil, then what can you do? We've been in some higher end Walmart locations and their parking lots were pristine.
More universal stores, like Kroger, vary widely by the areas in which they're located. Those in higher income areas have pristine parking lots to go along with the pristine interiors of the stores while those in lower income areas will have plenty of oil stains in their parking lots as well as less well cared for interiors.
The parts chains don't count since they are usually free standing small box stores that appeal to a retail market driving something old enough to need parts. Many also work on their cars in these parking lots to include oils changes and nobody worries too much about spilling oil on someone else's concrete, and these lots are usually concrete and not asphalt, the better to support jack stands on a hot day.