Many parks will charge double if you have disconnected the towed vehicle from the motorhome and then both drive in. That's considered two separate vehicles. If you leave them connected they'll only charge for one vehicle.
As far as driving in the camping unit - truck & trailer or motorhome and car and then also driving in a separate vehicle for siteseeing, then it's perfectly logical to charge for the extra vehicle. I see no problem with it.
As stated earlier, when folks try to cram multiple units on a campsite it gets out of hand. It affects your neighbor's views and space and also puts vehicles hanging out in the road or parked on the precious grass. That's another reason some parks require you to park only on the parking pad in order to avoid all these issues.