Just because one police officer doesn't say anything doesn't meant there are no laws or regulations against it, if there are laws or regulations it just means one officer either wasn't paying a attention or didn't remember the law.......
Yes: It's entirely possible that passing cops, rangers, etc... "just weren't wise" to some minutia in the law. And that a generation of md'rs were/are "just getting lucky". Sure, but is that the realistic observation ? No.
Example: if you're walking through central park in New York, you come upon a large pond, where scores of people engage in the past time of RC controlled miniature boats. You learn that this is an every weekend scene, for decades now. You think to yourself: "Gee that looks like fun. I have a toy RC control boat at home, so I think I'll join all these folks next week". Notice that it NEVER WOULD CROSS YOUR MIND : "Gee, I wonder if this is legal?". Why? Because subconsciously, you assume it must be ok, if all these other people are doing it.
So too is the same psychology played over and over on md'ing forum: Someone is going to travel to a certain area (beach, park , state, city, etc...) and inquires ahead : "Is md'ing allowed at such & such spot?". Notice what often happens next: Locals in that area chime in on the thread and say "sure, we detect there all the time, come and join when you travel through", etc.... However, notice, the person answering in that fashion did NOT say: "I asked permission, and was given the green light". Instead, that person answering was probably just going where his mentors before him went (why would he question that?). In the same way as the toy boat people in NY central park (why would anyone question that?)
So on the one hand, you're technically right that just because a place is routinely detected, and no one's ever cared, does NOT mean that you could possibly find someone there to say "no", or dis-like it, etc.... Yet *realistically*, we all subconsciously make our decisions based on what we see as normal, via our eyes looking at every-day-events.