Fripp Island was inline with discussion on the privatization of beaches. The only bridge to get to Fripp Island is through Hunting Island, and they don't let non-residents through. Sea Pines is a little more lax since it's not as elite as Sea Island - they don't check the credentials of cyclists, and it's the only way I've visted.

I'm not sure how much of the beach is considered public in east coast states, I know CA has laws against full privatization. But by gating the only road access, you've effectively privatized the beach unless you kayak there or something.