RVs in Parking lots

Have you ever noticed those big RVs that camp out in parking lots like Wal-Mart and stuff. What gives? If they can afford the big monsterous RVs they should be able to afford a campsite at a campground or something. It's ridiculous especially when they take up several spots from actual paying customers.. Yes, they might go in there once or twice if at all. But if their just their to camp then go to the very far corner of the lot that's not used unless it's Christmas time and everyone's at the store..