I think that's pretty much standard operating procedure for lawyers, isn't it?

Would you site a law detrimental to your case?
Good one. lol Unfortunately that is standard operating procedure for most litigators, and it drives me crazy. I have always felt the best way to handle case law that works against your argument is to go ahead and get it out there so at least you can "manage the discussion." Don't let the other side enjoy any gotcha moments, etc.
On Permission. I definitely understand the feeling that laws
forbid activities or
add conditions to something we're already allowed to do. After all, that's our day to day experience with laws, they usually tell us what we can't do or how we can't do it. And because of that, it's totally understandable to think that we members of the public have a right to do what we consider harmless activities at a government park so long as it's not specifically disallowed.
A lot of us, myself included, refer to "public" and "private" lands, but maybe it should be clarified what we mean. In everyday conversation, "public" or "private" land just refers to access. A city maintenance yard is usually private, and a retail store is public. I cannot think of any example of property literally owned by the public other than in the most theoretical and impractical sense.
Almost no private individuals have gone through the trouble to construct a park, a parking lot, and a sign reading "PARK." Cities have. So I guess that's why we (at least I) often refer to public vs. private lands, but what I really mean is public-access vs. private-access lands.
But suppose I do construct a walking trail on my land and post a sign near the road stating: "WALKING TRAIL / OPEN TO PUBLIC." A court would say that I have now extended a "license" to members of the public permitting them to enter my land and walk on my trail. However, the court would say that strangers would not be allowed to fish in my pond or sleep on my trail. I would not have to post a rule against fishing or sleeping; I simply never gave anyone permission to fish or sleep there in the first place. I extended a
limited license. It's the exact same thing with city property. Land owned by the city does not have any weird underlying special characteristics. The city owns the property just like a private individual owns property. The difference is that the city spent the time and money to set aside part of its property and develop a public park. It's still privately-owned property; it's just accessible to the public. The city owns the land and has the right to do whatever it wants to do with it (with irrelevant exceptions). The city can prohibit everyone from trespassing on the land just like I can do with my own property, and the city can revoke someone's permission to be there at any time and for no reason.
However, I know that's not the argument of someone like Tom (right, Tom?). He will correct me if I am speaking out of turn, but I believe his opinion is that metal detecting is an activity that falls within the license granted to members of the public by the city. That's a really fair argument, I think, and I would go to bat for that argument any day. But for me, and Tom basically said this, I'm the kind of guy that needs something a little more definite. Even if I can just get the city hall receptionist to say that she's sure it's fine as long as I'm not digging up holes and leaving them and carrying a big shovel, at least then I could point to some sorta-kinda "permission" in case of a confrontation in the field. I agree with Tom that it's not really a legal thing. My secretary excuse is not going to win any argument with a cop, although I think it might persuade a grumpy old lady to leave me alone, or it might just help me "save face" with a cop. It won't yield any substantive wins or losses related to MDing privileges.
The whole question of whether MDing is presumed to be allowed or requires special permission is interesting, but unfortunately there can be no bright line answer. If there are no rules for a city park, then the terms of the license to use the park are ambiguous. A court would say that the license is defined by a reasonableness standard. Would a reasonable person think that metal detecting is allowed? What about skateboarding? A historical society curmudgeon might say that metal detecting is "clearly" against the nature of the park, whereas a helicopter-parent soccer mom might say that skateboarding is "clearly" against the nature of the park. A jury of peers would theoretically have to decide. I think both activities are probably included within "park type activities," but my opinion might not be considered reasonable by those in my subculture. Here lies the unpredictability that has been discussed at length. Ultimately this is a fruitless exercise, though, because the landowner always has a right to revoke the license whenever it wants. Hopefully everyone understands that a landowner can do that, whether it be a private residence or a public park. That's why, for my comfort level, I would like permission so I avoid wasting my time or embarrassment. I recognize that not everyone would be embarrassed by being turned away by a cop in front of a bunch of fellow citizens, but I would. To each his own.
On Easements. In common law states, easements are just permissions. Property ownership is a bundle of sticks. One stick is the right to inhabit the property. Another stick is the right to exclude others. Another stick is the right to mine the minerals. And so on. An easement is a just another stick that has been given away entitling the stickholder to use the property for something particular. I can grant an easement on part my land to my neighbor so he can drive over it to get to his "back 40" without being a trespasser. Before I even bought my property, the prior owner had [been forced to] grant an easement to the utility companies to run utility lines across the first X feet of the property. My front porch is on my property, and most of my driveway is on my property, and then further away (but still on my property) is the boundary of the utility easement, and still further away is my property line. My driveway continues beyond my property line and connects with the road for my own egress/ingress. The sidewalk in front of my house happens to be slightly beyond my property boundary, but my neighborhood covenants (which bound my parcel before I bought it) required that I pay for and construct the sidewalk the full width of my property. The sizes of easements and the convention of whether sidewalks are on private residential property or city property vary by jurisdiction, but the general meaning of those terms is consistent among the U.S. common law states (so not LA).