" .....but there are also examples where people get in trouble if they detect in certain places."
Yes, but why are they "getting trouble" at certain places? Whomever came by to give them the boot, might be persons "appraised" of how "you shouldn't be doing this", because of the rabbit trail of evolution I speak of. So you have to go back further, and ask "how did it start" (in other words, why is someone being booted, to begin with?).
For example: there was a booting in a state campground in my area, where, previously, detecting had simply gone on, and no one cared (as long as you weren't being some sort of nuisance). However, this changed one day, when a friend of mine got booted by a ranger he'd seen before, where nothing but a passing friendly wave had ever been exchanged. Later, this friend of mine found out that a week before that occurance, a newbie had taken it upon himself to ask at the kiosk "can I metal detect here?". The clerk was confused, and had never given this matter any thought before. It was a question they simply didn't know anything about. But no problem, they pick up the phone, call various other supervisors, consult their books, and apparently find something to morph to apply to this "pressing question". (cultural heritage stuff, or whatever). They tell the fellow "no". Well gee, then guess what happens when this same clerk sees another md'r out there (whom perhaps he previously would have have given a moment's thought to?). He'll remember the earlier inquiry, and start booting others! This is just one example. These type "BOL's" and "memo's" to rank and file, DO happen, following such well-meaning inquiries.
Sure, I realize it sounds like a "catch 22". Because, how is someone supposed to know if it's allowed or not
UNLESS they ask? Right? The answer is simple: look it up for yourself. If there's nothing specifically there saying "no metal detecting", then presto, there's your answer

That way you don't kick a hornet's nest t make yourself a bullesye for scrutiny. If someone still comes up and says something, you merely tell them you read the rules, and saw nothing that said "no metal detectors". If they still want to push the issue, so be it. If this really bothers someone (that not everyone will love their chosen hobby), then I suggest to that person that perhaps they've chosen the wrong hobby. Because let's face it: we're in an odd hobby that has connotations, draws lookie-lous, etc.... There's no getting around that. But I maintain that you and I will only make it worse for ourselves, by going around grovelling, asking for sanctions, blessings, etc....