It is the same practice in our provincial parks. There is a price for "day use".
The parks site price includes...site, one vehicle, one boat and six people (unless your immediate family is larger) You can have your RV ( or main tent) dining tent and one other tent. If you have guests then there is a charge for them and their vehicle, still no more than six per site.
When you register you have to give everyone's name and address that is on the site and any guests have to be registered the same. They will not give your site # out to anyone unless you have given permission to do so. (great for not getting unwelcome guests, LOL)
I think it is necessary for extra charges for extra people and their vehicles.