Society has shifted towards a "me first" attitude and there are many who think they do not need to follow rules or common courtesy. Some of these people are pet owners and they will not pick up after their pets. They figure, I paid my fee why should I have to work while I'm here. As more of them are seen not picking up more people will decide: if they don't I don't need to either.
CG owners (trying to make a living for themselves and pay their employees) have few good options to deal with the mess left by the self-entitled:
live with the mess and listen to campers complaining about the mess left in "their" CG site (as if they now own the site)
eat the costs of cleaning up (or raise fees on all)
ban or limit pets and eat the costs of cleaning the reduced mess (or raise fees on all)
charge for all pets to cover the costs for the clean-up left by some pet owners
pay employees to police & fine (or eject) eating the costs (or raising fees on all)
reward other campers to act as police (campground nazis) and fine (or eject) the turned in rule breakers
As consumers we have the choice to patronize those CGs that have chosen our preferred means to deal with those not willing to pick-up after their pets.
We are paying for the privilege of being guests on someone's property (and other services they are willing to provide to us). Try acting like you would want guests to act on your property.