spisswgt, I recall from other posts of yours that you are fairly new to this. Herein lies the problem with the wonderful internet age: Persons will read a web-link like that, and assume that archies and laws lurk around every corner. No matter HOW isolated those incidents might be, the human mind thinks "oh NO! I wonder if there are laws in
my area?" and they scurry away in fear. Then the whole cycle only becomes more self-fulfilling, as people start inquiring of other authorities at different levels "just to be safe". And then you have various levels of city, county, state, fielding inquires about whether or not detecting is allowed. Bureaucrats, who quite frankly probably never even cared or would have noticed, now needing to address the issue. And we all know what the easy answer is. Or maybe there might actually have been something about 100 yr. old items, or disturbing the vegetation, or ARPA, etc... but it was never an issue, and people would go un-bothered (till you asked).
Example: I recall when FMDAC was first getting started back in the 1980s (before the age of the internet). Our club got on their newsletter mailings, and began to receive their periodicals. This was the FIRST anyone in our club had EVER heard of anything about laws, restrictions, and stories of people facing tickets or penalties, etc... I still recall other more timid members reading these accounts thinking "oh no! I wonder if I'm allowed to detect in the park
here, or the beaches
here, etc..." But what was wierd is, prior to receiving these FMDAC items about isolated incidents in other states, it had never occured to anyone here, that there was any problems. We would just go to
any park, school, beach, etc... and detect. Afterall, they're public right? I mean, where ELSE are you supposed to detect?

It never even occured on us that you might need to ask, or that there might be a rule, etc... (barring obvious historic landmarks, or leaving a mess, etc....). And no one ever had any problems (why should they?). Now though, reading these FMDAC mailers (which is similar to your website link stories) people started asking more questions. Either it was along the lines of "permission" ie.: "can I have permission to metal detect at the school, or park or whatever", or along the lines of "are there any laws to prohibit metal detecting, or do I need a permit", etc... Oddly, people would sometimes get a "no", at places we'd already been detecting for over 10 yrs! Yup, detected right in front of anyone and everyone, with no prob's. Now someone in our club comes and tells us that such & such park is off-limits

And of course, some people just said "nonsense!" and continued to go, and continued to never have a problem. All I can figure is, if you ask enough bureaucrats if you can detect (be sure to show up with a shovel in your hand when you enter their office, lest they not fully understand the implications, eh?), you're bound to eventually find someone to tell you "no". But I bet that same bureaucrat, would probably never have paid any mind if he'd seen a random md'r out there, had he not been asked to render a decision. Now that he's told one person "no", guess what he'll do when he passes
another md'r, who he previously would probably never have noticed? He's going to start booting others! (as in "aha! there's one of
THEM!")
So this whole thing has only fueled itself, as people read about what probably was, way back when, singular isolated incidents, or very isolated city or county that had/has a permit system. All it takes is only one incident or permit, and when someone else 3000 miles away reads about that, they think "aha, I better check my town/city/school/park, or whatever. But you can see that if he'd had no knowledge of these things, he'd probably never have thought about the matter to begin with, and simply gone out to detect.
So all I can say is, maybe it's better not to read those things. You know, like someone who watches too many cop reality shows, probably walks around with more of a fear of muggings or assumptions of wide-spread crime, etc... Same thing for us all to sit around reading sites like that, only puts us in the fear mode, which only becomes a self-fullfilling prophecy.
And as far as ability to keep an object over 100 yrs. old, HECK, I bet that "technically", there are lost-&-found laws that keep you from keeping anything you find on the ground, even eyeballing it. Ie.: if you find a valuable item (like a ring, or wad of $100 bills, etc...) there is probably municipal code everywhere that would require you to turn it in to the police, and wait 30 days to see if someone claims it, etc.... So those laws could, if you tried hard enough, be made to apply to every single coin you find. I mean, afterall, you found it on CITY property, therefore it belongs to the city, right? So as far as the 100 yr. rule, your math never was too good anyways, right?