i find swan pooh, and rabbit droppings much less offensive than the steaming stinking piles left behind by meat eating doggies . I also find the quacks of ducks much less annoying than dogs barking when their owners have gone out and left them behind only for the owners to deny that theor dog(s) bark.
When i visit some sites with a heavy dog presence you can actually smell the urine and pooh, I have seen pitches where the gras has died back in patches due to dogs urinating all over the pitch, so dog only pitches sounds like a good idea.
As for charging for dogs, whats is the problem ?
These sites and their owners are running a business, they are not charitable organisations, or funded by the public purse soley for your pleasure because you have a dog(s).
And dont forget , that many dog friendly sites have set aside a field or other area for dog walking and have to pay rates or rent for that piece of land. The owners may have to cut back grass and weeds , that has a cost attached to it. There are fences to erect and maintain, dog pooh bins to erect, maintain and empty, some provide bags for the turds.These things are not free.
If you dont like what the site charges for your dog then dont go, chose another site, there are others.