I'll be serious here for a minute and say that It is a moral dilemma. Had you paid, they would've figured that it was past the date the sign was posted that the CG was closed. Who knows what would've happened. I've read some pretty crazy stuff lately on what police are doing to enforce federal areas that are closed. Yet, the host knew you were there and didn't let you know, or worse yet, the federal employee posting the sign didn't do their due diligence and check the campground. What's done is done, just leave it and move on. As one said here, there are bigger things for them to worry about.