You can always find an outlier - OW. I still call it a bullshit scenario. And the fee is just one part of the cost to visiting a park.
The poster child for these fees is Yellowstone, which was raised to $20 a few years back, and may be even more now. The population of Wyoming+Montana is fairly low, with the vast majority of visitors travelling hundreds of miles to visit. Consider a resident of King county - the trip is at least 600 miles to the N. entrance via the I-90. Even with a car with good mileage (25 mpg), that's 24 gallons of gas each way, or 48 gallons round trip. At $3 / gallon, we are talking $144. A $20 entrance fee is around 15% of that cost. Now add in camping fees - at least $10 a night for a semi-developed site (I just paid $12 in the Olympics at one site, and $18 at another). Or does this poor family just poach - there are issues with that as well (damage to the undeveloped sites, garbage, etc, and extra cost commuting daily into and out of the park).