I'm not a campground owner... but in my mind this is a poor policy. If you are checking in 2 hours early, there should be no charge if the site is empty and the campground doesn't have to do any cleaning or anything.
Now, if check in is at 3pm and you show up at 8am, I could understand charging you $10.
We actually like Gettysburg Campground and are going there in 5 weeks. We generally get there around 2 or 3 and have never had any problems. Its frustrating to hear that this is an issue... hopefully its a new employee that didn't know they had some discretion in this charge.