Claiming a first come campsite

Pulled into a campsite at Mezadian P.P. Paid our fees for a few days and thought no more of it. The next day as we were still in the campsite a couple drove up and said that the site we were in was theirs as the night before they had left a couple of chairs at the site. I said, no chairs were here, we paid our money and were staying. Right then a park employee showed up. She had the chairs. She agreed the chairs were there yesterday but they did not pay for the site! So, no money paid, no site saved. But, they said, they were going to pay but went to Stewart and decided to stay there so did not get back to pay. They thought the site was still theirs. They got their chairs back but that was all. Sheeeesh!