Forest Service Campground - First Come, First Serve & hosts

We were in California, don't remember where. Looking for a spot. Campground had plenty vacant sites. We called for reservation headquarters said no vacancy. Checked with camp host,He pointed to an empty site said that is in use for a week but the people packed up go ahead. We set up fully expecting an outraged camper to show up in the middle of the night. Nope, stayed three quiet days.
Did not pay either. Host said thesite was paid in full and the system would not accept payment on an occupied site. Felt a bit guilty but?