One of the things that the Ohio state parks did in regards to people booking the entire week and then going in to modify their reservation just to get the weekend, was that they no longer allow any reservation changes. If you cancel the reservation it cancels the entire reservation. You cant go in to modify your reservation and cherry pick a few days out of it. After you cancel you need to wait until you can try to rebook it. In the meantime others can sign up for email alerts if a site becomes available in that campground. So the original person who made/cancelled the reservation stand a good chance that someone else already booked it.
At least here there are no more days of just packing up your camping gear and going to find a site that day. Everything in our state parks is by reservation. Even if you pull up and the CG is half empty you still need to go on the reservation system and reserve a site. They don't allow walk ins.
The days of not planning out your trips ahead of time for summer and making reservations thinking that you will just pull up and get a site are virtually non existent anymore, at least in summer at the popular campgrounds.
As far as what others suggested, that if they didn't show up on time the site should be re-opened. Who makes that determining factor as the 'why' they didn't show to see if a site should be reopened? What if someone was called into work, truck issue, trailer issue, kid broke arm..but they still intend on coming for the rest of the reservation..is it the 17yr old kid at the check in?