There are unanswerable arguments regarding charges.
A 1 man tenter may complain why he/she has to pay the same pitch fee as a huge caravan or motorhome. A motorhome owner may wonder why he/she pays the same as a caravan and car, yet is only 1 unit. Someone who never uses the site facilites may ask why they pay the same as someone who does. Should a disabled person pay more because the site has put in specific facilities for them?
Basically, the site has a package of fees and biases these towards the greatest cost base. It could be argued that providing a dog walk removes a parcel of land that could possibly be used for other profitable purposes, so should necessitate a charge. After all, many people who do not have dogs are not going to be inclined to use it for recreational purposes, for obvious reasons, therefore it is there for a minority. It is not the only reason, of course, but sites look at many things when deciding where to levy charges. For example, I have no idea if sites have to acknowledge the presence of dogs for their insurance cover, just in case anyone is bitten, or whether it is taken as read. Some sites, fortunately just a few, of course are just plain profiteering with their charges, but then again you can strike these ones off your visiting book.
Again, at the end of the day we all pay for our hobbies whether it be caravanning, or a love of dogs or any other animals, or some form of mechanical recreation. As some very stressed parents would possibly state, at least you can tie the dog up when it behaves badly. I just wish I could do the same to the b....y kids.