We stay mostly at private campgrounds and the majority of them have visitor fees, of varying amounts. Some differentiate between staying during the day and staying overnight. ALL of the campgrounds had a NO PETS ALLOWED policy for visitors.
With some of the campgrounds, it depends upon who the visitors are and how long they will be staying, and possibly if you're a "known quantity" or not. For example, a few times my mom & dad stopped by for lunch & some time around the campfire. Other than our RV, they didn't use any campground facilities. Some campgrounds just wave the fee and give me a paper sheet for them to put in their window. Others will charge the day fee. Some have given a time limit. For example, if they're visiting for more than an hour, then they want to collect the fee.
So, I've seen variations on the theme. I understand why campgrounds have to impose the fee and have been at a campground or two where it's obvious the "seasonals" are allowed to abuse the privilege. Kids (unsupervised) EVERYWHERE!!!
~Rick