Steve in Leo makes the point that sites need to make a profit. That is the underlying principles of business, and no site can continue if it does not have the necessary income cover its costs, and to set some funds aside for future commitments where suppliers prices are increasing and for maintenance and improvements.
I am sure there are some site owners who genuinely want to offer good value and who have are not looking for vast profits, but there will be others who are looking at purely from a business perspective where maximising profit is the goal.
As there are no laws or regulations that control what or how site owners can charge (with the exception of power hook ups) , then some will try to milk their customers by charging for every little thing they can. Previous threads have called into question the charges for dogs, and children, tents etc.
In all cases though, a site owner will charge what they believe the market will stand, and if they get it wrong, then at one extreme they will loose custom because they are too expensive, or at the other they will not generate enough cash to cover their running costs. Market forces.
As a customer, you are not forced to use any particular site, it is your choice, and part of the decision you take is consideration of the site cost against facilities or convenience. If it's too expensive go elsewhere.
Actually there is nothing to stop you from haggling to get a better deal. But do so before you go to the site and get it in writing, not when your leaving site.