What is wrong is that the sites are reserved by people not actually using them. It is not unusual for someone to reserve (and pay for) a site for a two week window in order to have it for two or three nights at the end of that period. For the people doing this, the value of those nights is sufficient to justify the cost of a two week rental.
It may or may not be against the policy of a public park to tie up a facility but not use it, but separating the reservation system from park management makes it just that much more difficult to police, where policy considers it abuse of the reservations system. And for some parks, all they care about is whether or not the site is paid for, and so don't consider it abuse.