That person who temporarily parked in your spot was fine. They apologized and quickly moved it. No harm done.
Those persons who tore up your ticket and took you spot should have been removed from the campground and fined.
I like the "must occupy site the first night" requirement. If that doesn't happen, it should be open season.
Years ago when our two daughters were quite young I left with them to set up camp early at our favorite lake. My wife would follow. The rule then was you had to occupy your site and fill out and pay within an hour. I got set up, had the envelope in hand and was waiting for my wife so we could put the fee and the extra car fee in the same envelope. I was technically breaking the law. A ranger showed up, heard my story and fined me! My daughters were watching the interchange and it took all of my reserve to stay calm. She fined me $30 and then said that I didn't have to pay the camping fee ($10) for that first night. I wrote a letter to the State Ranger and got a very nice reply with a promise they would talk to the individual. I did not get my money back though.