From the replies here, it seems the fee is working; some people with dogs are no longer going to the state parks.
We camp a lot in MD, VA, and PA. MD and PA do not charge an extra fee for dogs but limit the sites by location where one is permitted with dogs. This causes a problem for us as often, all pet sites are taken, often by non-pet owners. This is not a problem in VA. An extreme example is Assateague State Park which, until this year, had exactly six pet sites out of a total of 350 sites. One had to book almost a year in advance to get one.