I am strictly talking about private parks in my post.
I like visitors fees. If I have only my family using the site but my neighbor has tons of his "closest friends" using his site, then who is using more of the parks resources? If they are using more, shouldn't they pay more? The park has to cover their expenses so they have two choices. Raise all of the rates or charge for visitors. I shouldn't have to pay more because my neighbor wants to have visitors.
Charging only the people using the facilities doesn't make any sense. Every single person there is using the facilities. Sure, some visitors use the pool/playground but the others are adding wear and tear too. They are also probably using the bathroom and using water/sewer. Those things aren't free to the park.
I'm sorry but quibbling about a $3 or $5 day per person is sad. People always want something for nothing. My tax dollars do not go to support private parks. I pay a use fee. I am fine with that. Why in the world should a private facility let anyone use it for free? Just because you rented a site for the night does not give you free access to use the heck out of their property. Besides a public tax payer supported entity, where can you go and pay such a tiny amount to enjoy a place for the entire day? Can't golf for free. Can't watch a movie for free. I guess you could walk through a mall and use their bathrooms for free.