I believe it is free to anchor, but they'll still try and charge you. I've never had the courage to dispute it, or known anyone who has. Use their buoy, and yes they have a right to charge. Don't know if it's a NT member perk, though.
I take it you haven't heard about the deadly bug there that eats Larson hulls in a matter of minutes. Also the acid in the water that can strip the paint clean off Volvo DP-H outdrives in an hour.
Also the snakes, the odd shark, man-eating patches of quicksand, and a live firing range.