I agree that this is an extreme example of someone wasting the opportunity to use the site. With that said, keep in mind the opposite - we showed up at our reserved site at a local provincial park, only to find someone else had decided to occupy it. Their justification? "It's 8pm, we didn't think you were coming and our friends are staying across the way. Do you mind taking our spot in the back forty while we use your lake front site?" Ummm... yeah... I do mind, thanks. Had to feel like a jerk (not sure why) while they packed up their generator and brought their slides in, hooked up to their 5er, and got our of our site.
Anyhow, all this to say, maybe they want the freedom to come and go from their reserved site as they please. As others have said, they paid the rent, and get to do what they wish.