Same at Ponui last week. Old guy, small dog, dinghy pulled up directly in fromnt of the "Private Property, No Fires, Camping, or Dogs" sign. Off lead meander along the beach and back...
I think its a mix of causes. Ignorance, pets as substitute children, assumption of an inherent right to do what you like. Same stuff that drives other mildly selfish antisocial behaviour like parking in disabled carparks on on the grass berm (pet peeve).
Fish is right - confrontation is not useful. Education/Information is more useful.
Refering infringements to an appropriate authority is