You've gotten some good feedback so far (and some people mentioned my name, haha).
Kimber: the long and short of it is, it sounds like you were a victim of the psychology of: "No one cared, till you asked" routine. Notice, for example, that the clerk didn't even know the answer (a question she said hadn't come across her desk). So sure as heck she's gonna float your "pressing question" to legal department, and other such people who likewise perhaps never gave the matter thought before, nor would ever have noticed or cared! But the mere fact that someone calls or writes to ask, simply gives the implicit connotation that something is inherently wrong with your activity (damaging, illegal, or whatever), lest .... why would you be asking, to begin with, if there weren't something inherently wrong with it? You see how that simply dictates your answer? (the subconscious effect is not lost on the bureaucrat, who will simply give the easy answer, when in fact, he may never have paid you a moment's notice, if he were just passing by on the street)
Those clauses about digging, holes, vandalism, alteration, etc.... ALL clearly imply an END result. Thus if you leave no sign of your presence, then by logical definition, you have not "vandalized, altered, left holes", etc.... now have you? But if you leave that in someone else's mental picture (where of COURSE they're going to take the easy answer), what did you expect? You will never win that p*ssing contest debate of semantics when you go grovelling for sanctions. Because let's face it: no matter how you slice it, there's going to be the temporary evil process of extraction. Kinda like nose-picking: sometimes you just gotta be a little discreet, for pete's sake. But if you ask someone "can I pick my nose?" of COURSE they're gonna say "no".
Even you yourself admit that these parks have probably already been detected, and that they're innocuous, and that the answer was arbitrary. And now you're surprised?? Why didn't you look up the laws/rules for yourself? If there's nothing specifically saying "no metal detecting", then ... gee, I guess it's not prohibited then is it? And no, "destruction" and "altering" type clauses I do not define myself as. Because to do so (to make the automatic equivalence that detecting = destruction and holes) then yeah, you might as well give it up now. Because sure, ALL parks forbid alterations, destruction, and vandalism. And since YOU know you'll leave no trace, then why isn't that good enough?
I can think of many cities where detecting had simply gone on since the dawn of time, and no one ever had a problem (unless you were being a nuisance in some other way). But lo & behold, someone takes it upon themselves to waltz into city hall and ask. They get your response. They pass the word to the old-timers that "detecting isn't allowed". The old-timers are left to scratch their heads and ask themselves "since when?, who told you that?". You see how it starts? It's a self-fulfilling prophecy (the "squeeky wheel") that ends up getting policies and/or rules in place, to address the "pressing issue". In other words, guess what's going to happen the next time your city-hall people see another md'r in the parks now? They're going to remember this dialogue and think "aha, there's one of them!" and start booting others. I've seen this happen!
So next time look up the rules for yourself (easily available on city websites, or down on the desk in the city charter at city hall, etc...). If it's silent on the subject of metal detecting, then there you go. Naturally, just like nose-picking, pick low traffic times, don't hunt if busy-body gripers are present and studying you, etc.....