Well, granted, you're not going to be able to detect @ Coronado's Navy seal training beaches, Pendleton,or other such off-limits beaches. But , heck, that's not for md'ing , that's probably for any entry. You probably couldn't even park or walk on those secluded stretches. But it's sort of a non-issue, as those are the normal beach access points in the first place for the general public.
Actually, you can hunt some of those areas, as long as you can get on the base. (I.E. military I.D. or guest). You just need to know which parts are open and which are not. Half the time, the MP's don't know!
Yeah, this is not unique to there. Up and down Ca and other states, there's ribbons and barriers to keep people from trampling the dunes. So that , I guess, you don't disturb the nesting snowy plovers. Or hurt the fragile dune grass (heaven forbid). But again: This has nothing to do with detecting, per se. It's for all persons to "stay on the paths", till you've out into the beach. And scores of So. CA beaches don't even really have "dunes" anyhow. A ton of them have had historical municipal sprawl all the way up, so that the parking lots and streets and beach-side homes literally touch the beginning of the beach "towel line" area, it seems. So on those, you would run into no such nonsense on that wildlife type issues of dunes.
This is where we get run off because of ignorance and/or they just want to give you the easy answer (NO) and move on. Even though signs are posted to keep out of the dunes and nesting area are clearly marked, there are those who ignore those signs and hunt anyway. The rest of us get tarred with the same brush and denied even hunting the surf. They seem to tolerate the other beach goers though....even ones who walk into the dunes.
I'm sure those rules are so that they can reign in late-night college keg parties. Or simply that the parking lots are what closes for the night . But you can still walk from further away. We too have some beaches here that ..... if you looked long enough and hard enough there might be a "closed at sunset" clause ? Yet you will routinely see late night romantic couple stroll, etc.... If such hours are actually enforced down your way, then ..... so be it. But it would seem to be sort of a joke, if their way of patrolling/enforcing it, is to drive up and down the beach in an ATV. Because you could see those headlights coming from a mile away. And then simply walk into the shadows somewhere till they pass. I think they have bigger fish to fry than a man with a metal detector.
I'll just stay down here in San Diego where we don't have such nonsense as curfews and paid parking. I might get in trouble up there an La La Land. 
