I notice this rule applies mostly to "public" parks. Is it because of this public status that folks feel an overwhelming need to dictate exactly how the site is used? On the other side of this argument we stayed at Valley of Fire SP near Vegas this last spring. Their sites are first come first served. We arrived on a Wed morning and were able to get one of the last sites. Across from us was a family from Germany in a rental rv with no toad. They had driven into the deeper areas of the park for the day leaving behind a stroller to mark their paid for site. Can you guess where this is going? Some locals showed up and decided that since there was no RV in the site then it was free game. Ended up being an ugly scene. IMHO If a site is paid for then it is yours whether you park your rig, tent, or sleep on the ground; sure it is rude to waste the resource if you leave it empty for selfish reasons but that is another discussion altogether.