Some parks have a "Minimum reseveration length" for example. on a trip to NYC I opted to overnight in Jenny Jump (PA) State park, they have a 2 night minimum, I only overnighted.. Thus the site was only occupied the first night.
However I can see cases where I"d have to occupy the 2nd night of a 2 night minimum.
IF the site is pre-paid, then it should be mine for the duration... If it's only a deposit.. then the deposit should hold it only for a limited length of time. That is my opinion.. However in the case cited above.. If someone else took the 2nd night I'd not object.