I think that there are rather different models operating here.
Certain business, eg Starbucks, offer free Wifi to entice you in (after all, their coffee is no inducement).
This, and much of the other free or included in other cost services that Cris is talking about are usually in town centres etc. Popular with the business traveller.
The caravanner, however, does not necessarily want to lug a laptop around with them when they are out seeing the sights, they want to log on in the evening from the comfort of their unit. As yet, their numbers are relatively small, and the number of sites offering facilities is also small, so there is not much pressure on prices.
As more sites realise there is a market, and as the cost of provision drops, availablity will increase and prices will drop. As that happens, any owners who are tied in to a restrictive contract and high prices will find that they are making a loss, as those who really want the service will go elswhere.
There are, of course, those who find an unprotected Wifi site, either private or business, and quietly log on to it. This is illegal without the owners permission, as you are stealing their bandwidth; on a practical level, you could cause them to reach their download limit and have their service stop - not a friendly thing to do.