First beach to charge for access was Bradley Beach in 1929. It was descendants of James A. Bradley who put the rule in place. Bradley developed Asbury Park into the resort town it became and was also a supporter of segregation during post-civil war reconstruction. So it has racist, classist origins. My guess is it’s still a thing contemporarily because most citizens don’t question it and the towns like the tax revenue. But if you want to be cynical, there are still people who think it’s a good idea to restrict access to “certain people” (just read some of the other comments on this post referring to keeping “riff raff” off the beaches lol).