True story
I too hold fast to "finders keepers", unless it's painfully obvious who lost it (class rings, or when you're commissioned to do a posse-hunt, etc...). But to add fuel to the fire, consider the following true story:
There's a 1930s elementary school, in a city not far from me. One day, in the late 1980s or early '90s, a fellow was hunting the sandbox for loose change. Imagine his surprise when he pulled up a choice seated half! He couldn't figure out what a seated half was doing in a sandbox of a school which only dated to the 1930s, but ..... he wasn't complaining

Moving along, a little while later, in the sand, he pulls up a choice silver dollar from the 1870s! Now he's thinking "this is more than a fluke ! Something's going on here". He figures that perhaps the sand was brought in from the ocean to put into this sandbox (as they add new clean sand from time to time), and since we're not far from the beach here, these coins must've come in with the incoming sand. Wow!
He tells a buddy of his about his two choice coins. So a few days later, both of them go back to this sandbox, to try some more. And AGAIN choice silver coins: a bust half, and a seated quarter. They are just amazed. One of them wanders off into the grass, and finds another choice coin from the 1800s, that didn't even seem that deep. So now their minds are racing, dreaming up conjectures of how these coins might have gotten there. They reason that perhaps there was an old camp site, or stage stop or something on the site of where this modern school exists now. Afterall they reason, the old main road goes right past this site. And so on and so forth they dreamed up reasons for their good fortune. The two guys returned several more times in the coming months, and ........ all told ...... by the time they were done, had something like 12 choice bust and seated coins.
One day, one of the two men was out there again plying his luck, when an after school janitor saw him. The janitor came out to talk to the old man with the detector, and tells him: "hey, do me a favor: if you find any old coins ...... and I mean REAL old, let me know". The md'r fellow, w/o revealing any info, says to the janitor: "really? Why's that?". The janitor then tells him the story that a 3rd grade kid, who was a loner, had brought his father's coin collection to school, without his dad's permission, for show & tell day. Then during recess, the kid proceeded to pass out the coins to all his class-mates, to "make friends". Well the other kids in the class just treated them as play money, and thought they were toys (because they'd simply never seen anything like bust coins, silver dollars, etc...) Thus the kids, during the course of several recesses, apparently had just gone out, played with them, lost them, etc.....
A few days later, when the dad got wind of what had happened, he marched his son down to the principal's office. The principle, in turn, marched the father and son into that specific teacher's classroom, and called the entire classroom to order: The students were to return all the coins that had been passed out to them, in the previous couple of days. Well you can guess what happened: very few of the coins got returned. Most of the kids had simply lost track of them, claimed not to have theirs anymore, etc. The father got back some of his collection, but many coins went missing.
Now imagine this: You are the md'r listening to the janitor tell you this. All of the sudden, it dawns on you, these choice seateds and busts you're finding, are all recent drops. I don't know, in this case, if the md'r fessed up to the janitor, what they'd been finding or not, as I only heard this story 3rd hand.
Now put yourself in the shoes of the father who lost his collection. If that were you, and you got wind of the fact that some yahoos with detectors had retrieved the rest of your collection from the schoolyard, what would you say?? Finders keepers, or give it back?

Of COURSE you'd demand your coins back, if you could track down who these guys were, with detectors. And what if the finders claimed they were under no obligation to turn them in to police, because afterall, the face values were only .50c or $1.00, etc....? (below the $100 threshold). Do you think a judge would say "no, it goes by the numismatic value, thus the finders were under obligation to turn them in"?
And while it might be easy to think "yeah, but those showed signs of having recently been lost". In retrospect, yes, the story shows that. But in their own minds
at the time they were finding them, they actually had legitimate theories as to how
else they could be there, eh? Or how about beach storm hunting: We all know that during erosion, the clad can be right next to old coins, with no rhyme or reason to depth. I've found gold coins, for instance, after beach storms, that were right there with the recent clad (ie.: an inch deep, or whatever). Although it's unlikely, how am I to know if the coin didn't fall out of a bezzle the day before?
Thus the reason why the law makes no distinction even, of collectible valuable coins. And the reason why the law doesn't make a distinction on when YOU think it was lost.
Because if it was truly "finders keepers", then every thief out there, when caught red-handed with merchandise, can/would merely say "I found it". Thus the reasons for these laws.