This is just another revenue maker to charge for pets. Just like "resort fees" for BS stuff that most people don't use. I think it has very little to do with what they leave behind. I see plenty of what is left behind not picked up by campground employees either. And the not over a certain weight rule? So my dog shits bigger she's not welcome?
I've seen $2 to $5 per night for pets. This can add up to a significant increase to your stay. And for people like us who don't use the parks common facilities and we pick up after our dogs this is "messy" situation.