This is the way that business works though. The same argument to apply to say entry into a zoo. Why should a family of 4 pay more than a single person? They aren't actually causing any extra expense to the zoo. But it wouldn't be fair if it was charged per family unit.
This is the same for a camp site. You are paying for the facilities in the best way that the site owner can spread the costs. The alternative would be to charge a fixed unit price which would probably work out to be the equivalent of a 3 person family. So is this then fair on the single person who doesn't use any of the camp facilities, but can see the family next door taking hot showers in the morning and evening, using the play facilities, using the dedicated dog walk etc. If it was evened out, then I suspect that a family of 3 would pay about the same, a family of 4 or more would be slightly better off, and single campers or couples would have to pay considerably more. This may then result in them not using the site, which would result in prices having to increase, which gets you back to where you were in the first place, only without the smaller families who now can't afford to go camping at all.
Come the end of the day it's a system that's been worked by the industry since it started, and most people are happy with it. The only thing that I don't like is charging for an awning on a hard standing, but then I mostly stay at Caravan Club sites where you don't pay extra for awnings or dogs. The answer is simple of course. Review the prices before you book and if you object to them don't use the site. It's your option come the end of the day, no-one's forcing you to use these sites.
If you're in a club then you should get, or have access to, the annual finincial statement. This will then tell you exactly where all the money comes from and goes to.