Hi bunny-nugs! Love your input
Really ?

Then you've just single-handedly wiped out detecting at every single park across the entire USA, at all levels. Because I can gaurantee you that there's going to be verbage that either directly says "dig", or some variation that anybody could morph to mean the same. Eg.: alter, disturb, deface, etc....
And ironically, I agree with you that there would, in those cases, be no need to "ask permission". Because when you think of it, NO PUBLIC OFFICIAL can give you permission to "break laws", now can they ? I mean, if I walked in and said "Hi, can I break the speed limit please?" Or "Hi, can I cheat on my taxes?". Naturally they can't give you permission to break laws (technically speaking, but sure, it probably happens all the time in a "wink wink just be neat about it" type answer.
But the fact that a) detecting is commonplace at parks across the USA and b) that that happens both with and without permission, and c) that no doubt rules exist to forbid "dig, alter, disturb, molest, take , remove", etc... that therefore d) THEREFORE THOSE RULES DON'T NECESSARILY PRECLUDE MD'ING. And instead, are meant (realistically speaking) to address the end effect of damage (holes left open , etc...) or for people who think they can steal park benches and cut down trees for firewood, etc.....