Ok- your correct in mentioning that it can be argued that we are "taking, destroying, and looting the past." BUT: RELICS and HISTORY of some of the Greatest Battles fought and Battle Fields of the Revolutionary War in NEW YORK are paved over with Buildings and Asphalt when they could be in showcases in schools and museums; educating the younger generation of the History of our Country. IMHO there's no LOGIC or VALUE in that argument. I watched one day in horror as a Backhoe was preparing a brook for precast cement troughs to be placed in so to reduce backups. As the operator was removing dirt from the bank of the stream I noticed (five ft down from street level) that he uncovered what appeared to be a full stack of dinner plates. I motioned for him to stop and he did. I yelled to him, "There's dishes there that you're going to destroy." He got down from the machine - he looked - and he got right back up in the box; waving his hand to me as if to say "F it." Good thing for him that no ARCHY was around - that would have ended his $38/hr. job for the day!
Eureka, here's my answer to this: There are times when some entity makes all their parks off-limits to detecting. Particularly at state or federal levels (but rarely ever county or city levels). And you're right: While this may be a commendable rule for sensitive historic monuments located within that state's parks system (some parks have a "historic" theme afterall), YET What
possible frickin' harm could we do in the OTHER parks and beaches within that same entity's system ? Therefore, in your mind, you might agree with *some* places being preserved (shiloh, ghettysburg, bodie, etc...), yet you wonder why the utter waste in that ....... so too ....... do other places (innocuous beaches, modern clad and rings, etc...) get lumped under the same brush.
Or in your mind, if some place will be bulldozed to smithereens to make a library or a Walmart (hence destroying all "context") then what POSSIBLE harm can it do for a guy to ply the disturbed spoils after the workers have cut out for the day. Right?
And on the surface, everything you're saying makes perfect and total sense, and seems to fulfill their own stated reasons for denying md'ing.
HOWEVER, you must put yourself in their shoes. The best way I can put a face on that, is to bring you back-in-time 30+ yrs. ago, when the beginnings of all such things were getting started: There was a book put out, called "Treasure Laws of the United States", by a fellow named Doc R. Grimm It had all 50 states listed in alphabetic order, detailing what those state's laws were, for their state's parks (just state parks, not city or counties). The idea Grimm had was wonderful: the user merely flips to the state he's getting ready to travel to (like RV travellers going state-to-state, for instance), for immediate reference to yes, no, or other such details, rules, etc... Great idea, right?

And when you turned to each state's page, Grimm had the re-printed xerox answer from those states, right on their own state's letterhead. Woohoo, now you just show that to any busy-body who approaches you, right? (can't argue with answers straight from the state's capitol, if they've said "yes", now can you?)
Here's how Grimm compiled his book: He merely sent off 50 xeroxed letters, to each of the state's capitols heads-of-state parks dept's, asking to the effect of:
" What are the rules regarding the use of metal detectors in your state's state parks?" And on the surface, that too seems like the best way to go about it, right? I mean, doh, WHO BETTER to ask than the state's themselves right? Again, a marvelous idea, right?
And then Grimm would merely print each of the 50 replies in his book. But here's where it got ... uh ... interesting. While some might have said "go for it" or "no restrictions", etc... . Yet the vast majority were riddled with dire sounding restrictions, or out-right "no's". And oddly, some of those "no's" and/or dire-sounding-things states, were in states that .... quite frankly ....... up till then, you could detect their state parks, and no one had ever had a problem before! (barring obvious sensitive monuments or whatever).
So you put yourself in the shoes of the person on the receiving end of an inquiry like that Eureka, and be honest: What's the easy answer? Because in EVERY of the 50 states, there's bound to be *some* that are of a sensitive historic monument nature. RIGHT? (or golf courses, or something no one wants you tromping on, etc...). So think of it: there is simply NO WAY that person answering can go into super minute detail and say "yes at these 52, but no at these 8. And "yes at this one, but avoid the historic cabin on the north shore, stay back 20 ft. from the edges of that". And ...... you see how endless it is? So seriously now, what's the easy answer? NO. Then there's never anyone pushing the limits, or rationalizing "gee they're bulldozing it anyhow" or "gee this park is only 49 yrs. old" and so forth. Just endless semantics someone must deal with, so the much easier answer is "no". EVEN THOUGH YOU'RE RIGHT AND A WHOLE HOST OF PLACES IT WOULD DO NO HARM.
So in that case (using the Grimm's book evolution/story), where did the silly-ness begin ? WITH THE ASKING! I still remember that era (as I've been at this for 38+ yrs.) and you had old-timers scratching their heads saying to themselves "since when?". But putting yourself into some desk-bound bureaucrat's shoes, can you blame them ? No.