olfacere, let me start by saying I totally agree with you. I too think that those artifacts look JUST FINE on yours and my's mantle place

And if the public thinks they are being deprived of "knowing about their past", they are MORE THAN WELCOME to come over to my house, and view it were it's on display in my mantle place collection trays, doh! Or they're welcome to buy some of them when I list my duplicates for sale on ebay, doh!
So when you see me post the answers the archie's would give to those questions/objections, I'm merely telling you how they approach and answer the questions.
As for your last answer, .... again, here's their logic: Let's say if you could get them to agree that there's no way in h*ck that targets out in the middle of nowhere, or on mundane beaches, or silver coins that only date to the 1950s, etc.... are ever going to be dug by archies in the future (a realistic archie might concede your example of the pyramaids in Egypt, that there's countless other targets in the desert elsewhere that a hobbyist could hunt for with no harm, that will NEVER be known or cared about, etc...) Or that the common sense of it means we should like-wise leave our trash lying about as well too, etc.... Let's just say you got an archie to agree with that. That probably wouldn't be hard. The TROUBLE is (and I'm sure you would agree), that if the govt. agrees to allow us to start hunting on spots supposedly off-limits now (take .... federal parks for instance), that you and I know FULL WELL that we'd be continually "pushing the envelope", right? I mean, for example, you and I would probably agree that *some* sacred monuments *should* be protected, right? You know, like Shiloh, the pyramaids, Bodie, Ghettysburg, etc... So let's say that even md'rs could agree on that point, that no one would argue that even those spots should turn into a free-for-all, right? Then here's what you're left with: Infinate middle ground semantics debates on exactly what is innocuous and harmless, verses that which actually truly might be dug in the future, and thus preserved.
So put yourself in their shoes. Even though admittedly probably only 1% of the real estate in federal parks might be the exact spot of something truly sacred from a historical perspective (and the rest just mundane hiking trails, campgrounds, beaches, etc...), yet it's just un-realistic to think they could/would start to try to differentiate between zones, targets, etc.... You know, like: "it's ok at the ball-diamond, beach, etc... at this park, but just keep a 50 ft. wide berth of the historic cabin on the north end of such & such park", etc... You KNOW that's an impossible task to try to police, figure out, etc...
So their easier answer is just to say "no" to any and all places you ask about, just to "keep is simple". And yes, their talking-points rehearsed answer is going to be about saving for future generations, etc.... And while you might get them to agree with your logic, they subconsciously "can't go there", because to do so, just opens up this pandoras box of endless semantics.