Again, we're talking about publicly owned campgrounds that are there for The People to enjoy - not privately owned for profit campgrounds.
Therein lies the difference.
Whether it's Yellowstone National Park or Catalina State Park, it's there for the enjoyment of ALL people, not just the ones who can afford to toss money around.
As for someone booking the site and dumping an RV on it for a week, that is not occupying the site. Campgrounds with the "must occupy the first night" rule mean occupied by a person not a vehicle or trailer.
Basically, if you aren't going to use it, then leave it for someone else to use.