As a dog owner, I also would be upset at the fee. Not because of the amount, but because it was hidden and not disclosed upfront. It would be the same as making a reservation at a campsite but hidden in the terms and condition was a $40 fee if you use the electricity.
Pet owners are generally a very responsible group, but like everything, there are those few who are so full of themselves, they hurt all pet owners through their irresponsible actions. We've all seen the aftermath of those irresponsible pet owners so we cant; deny it exist. So if the campground's only real ability to curb these bad acts is to price all pet owners out, that's life; just put that price upfront so we know ahead of time.