At a guess, a landowner doesn't want archaeologists forcing their way in to dig up their garden and place restrictions on future works on the property.
Interesting how the article says they're puzzled why he decided to be anonymous, then proceeds to mention that what he did is highly illegal in Ireland. It seems pretty obvious why he wants to be left alone...
I live in rural Ireland, and I've heard more than one story about a farmer who discovered some archeological find on their land and just plowed over it to avoid the hassle.
It's the opposite of what they want. Artifacts in isolation are not very useful to archeologists. They want them "in situ".
For instance, digging up a graveside and pulling all the artifacts out, without extensively documenting every inch of what was found at the graveside even uninteresting things like arranged rocks, destroys the context that might have yielded insights about burial customs.
Archeologists prefer everything to be left alone undisturbed, and generally the idea is you only go looking for artifacts when trying to answer a specific question, or if the site is going to be destroyed anyway for construction. From their perspective treasure hunters are going out there and destroying the record of history.
I see this repeated here in the comments, but it doesn't make much sense to me. The site will still be there to be dug by professional archeologists. Pulling up a few artifacts do not destroy it.
The location and surroundings of the artefact are a key part of what makes it interesting.
Consider: a painting restorer would be happy with a discovery of an old painting, and will eventually be scraping and polishing and removing layers from it to reveal the full picture hidden under the dirt and rot of the ages (carefully, a tiny bit at a time, after they've recorded what they're starting with, and recording everything as they go).
But if you just scrape random flakes of paint off yourself and send them over with no other information, they will be mostly sad and hardly happy at all. The paint is important! - without the paint, the painting is nothing; but the flakes of paint are not in themselves valuable.
The location and surroundings will still be there, just as they've been for centuries. They can be investigated and dug by archeologists. There is a much higher risk that interesting archaeological sites get disturbed by construction, farming, and nature herself, such as floods and tree roots.
A metal detectorists do not like unnecessary digging, so you can be sure that they will retrieve their objects of interest with the minimum upheaval necessary.
By "the location and surroundings" I don't just mean the farmstead or whatever. I mean things like the cubic centimeters of dirt immediately around, above and just below the object, recorded in meticulous detail. You know, all the stuff you've just dug through to get to it. People elsewhere are complaining about how long digs take, but have you ever seen an archaeologist's toolset? They "dig" sometimes with a sharp flat trowel but mostly with a tiny little paintbrush. The "dig carefully" of a hobbyist spending a fun couple of hours on a Sunday with their metal detector and shovel is not the "dig carefully" of an archaeologist with their six-month grant, tiny trowel, ruler, calipers, camera and 1/4" sieve for the dirt.
I can see how this might seem like an obvious solution at first glance, but all it would do is make the problem of amateurs destroying the context of finds worse, and by a lot. This kind of incentive will provide for many artifacts and little knowledge, as most artifacts are useless without the context in which they were found.
It is of course appealing if you think that all an artifact is good for is looking nice in a glass case. But if you want to gain knowledge about the past, this would certainly be the worst policy you could implement (apart from obviously insane ones, such as a bounty for destroying artifacts on sight, of course).
But I already wrote an extensive rant about this in another comment, which you might find interesting if you are curious about the topic. If so, sorry about the bad writing style and potentially divisive rhetoric, I'm supposed to be working decided to take a quick peek at hn where I found some discussion about a pet peeve of mine, which is of course a recipe for disaster.
