There HAS to be more to that story and pix. For starters, the term "public land" in the pix dossier next to the "priceless historic chain and nail junk" is no doubt referring to some form of federal land covered by ARPA. Ie.: something probably taken from a sensitive historic monument on a federal park. So the term "public land" in the context of where this quote on that dossier was pulled from, is specifically for something @ a federal level. NOT state, or county, or city level lands. Unless those lands too had something about cultural heritage.
Whatever it is that is hinted at (whatever miscreant, gasp, dug that chain fragment), has to be a fluke, and not the norm. I mean, this had to have been some dude sneaking a historic monument, being a nuisance, couldn't take a warning, or in SOME WAY "could have known better".
For example: the state of CA also has minutia in their state park's verbage, about cultural heritage (boiler plate stuff that every state's state park has I suppose). And yet, you can hunt our state park's administered beaches till you're blue in the face. And (gasp) might even find a buffalo nickel, etc....
So random pictures like that do not mean much. If you got enough purist archies in one room, to assemble a display in a museum, you would CERTAINLY come up with something scary like that. But what did you expect to come from a purist archie ?
It would be sort of like asking an animal rights wacko advocate from P.E.T.A.: "Can I leave my pet bunny in the car while I run into 7-11 to get a slurpee?" What do you the think they would say ? They screeech: "NNooooo! You can be arrested for animal cruelty! The bunny could suffer in the heat in the car! Your car can be confiscated and you can be arrested !!". But seriously now, what did you expect to come from a PETA animal rights wacko ? SO TOO do I put little stock into what some purist archies say.
As for some isolated example someone can find of a horrible penalty (jail time, etc...) for detecting, I would say that flukes exist in every arena of life. But none of us tends to let our lives be driven by "fluke stories". Example: I'm sure that if I looked long enough, I could find a "horrible story" of a motorist being pulled over and roughed up by an over-zealous cop, for nothing but a tail-light out. Yup, jailed, car confiscated, ticket, arrest, etc... for nothing but a tail-light out, or radio too loud, etc.... But does that stop you from driving ? Are you afraid to drive now ? Or do you write that off as a "fluke isolated story?" See what I mean?
I'm not saying to "throw caution to the wind". Of COURSE avoid a) obvious historic sensitive landmarks and b) purist archies [and their wonderful museum displays].