Great diaologe. I do want to understand the mindset, so bear with me a bit more:
I don't think California cities are any less off-limits than any of the other states. I suppose if I went around asking, I'd probably get as many no's as east coast states. But if I/we are not bothered, why would we start asking now? And sure, I occasionally get told otherwise. I simply move on, and avoid that one park, or one gardener's shift, etc...
Here's an example: When the internet was young (mid '90s), Treasurenet was the only forum out there (at least that I knew of). One day I logged on and saw a post from someone who was coming out to CA, and wanted to know if there were any rules or laws about detecting state beaches. As I read his post, I could see that someone else had already answered his question. The person answering had merely gone to a book called "Treasure Laws of the United States", and quoted the portion dealing with CA. It had some pretty glum things to say about the state beaches here, almost to the extent of forbidding it. Like "alert the ranger upon coming and going", "turn in all jewelry to lost and found" and "if you find an old coin, flag the spot and alert an archaeologist", blah blah blah. I was blown away, because at that exact moment, some storms were hitting the beach near me (Asilomar) and we were getting coins back to the 1800s, jewelry, etc.... The storm had damaged some concrete stairways, and each day, for a week, rangers would be right there, attending to the repairs, etc.... and we'd be detecting right there, in full view of them. They never paid us any mind. I mean, I had no idea of these rules, and it never occured to any of us that the beaches were just the place to hunt, esp. after storm erosion.
Now perhaps those rangers just weren't aware of minutia concerning detecting. Afterall, if no ones ever asked, odds are they've never given it a second thought. But I can assure you that if I had asked "can I metal detect here", IF they had gone into the minutia, in order to find the answer to my question, perhaps they'd have said "no" or whatever. Why would I want that? They have simply been detected here since the dawn of detectors, and why should the current year be any different to start asking now?
So all I can say is, I suspect that this same mindset is probably prevalent all over the USA, where....... no one cares .... till you ask, and then Presto! the parks are off-limits. I'm sure that this doesn't fit the evolution of all off-limits parks, but I do wonder how many got the scrutiny for just this type thing, whereas before, it was never given a second thought.
I mean, lets say for example that you are unaware that a particular set of picnic tables were reserved, in advance, for a group event. Since you were unaware (and had no reason to suspect, and thus no need to "ask"), you started to use the picnic tables for your own family's picnic. The ranger comes up to post the sign "reserved", and finds you using it. He informs you "sorry, but there is a group that's already reserved this table, they'll be arriving shortly, so you'll have to pack it up and move somewhere else". If that happened to me, I would say "ok" and leave. Would the ranger arrest me? Confiscate my picnic basket, etc?? Of course not! Would it make me think "aha, therefore, wherever I go, from now on, I should ask to use picnic tables?" Of course not! Is it reckless to have not asked before using that table? Of course not! Why then, is detecting thought of as somehow inherently evil? If someone has an issue, they're welcome to tell us, as in the example of this picnic table rule.
If there are any examples of someone hunting a park (barring historic monuments, posted sites, etc...) and getting arrested, ticketed, confiscated, jailed, etc... I would say that is the extreme exception. I mean, sure, sometimes a driver gets roughed up by an overzealous cop, for a simple lane change violation, tail-light out, etc.... Does that stop the rest of us from driving? Do you go to the police department for permission to drive?
As I think back on the 10 gold coins I found, I bet I'd have maybe only 1 of them, if I took the stance that some folks have on cowering for permission. I mean, take the ones found on the beach after storms. Gee I guess they qualify as "old" coins, and according to the example I give of CA rules, I was supposed to turn them in to an archaeologist? Or the $5 I got from a city oldtown sidewalk demolition, I was probably supposed to have asked at city hall first? (afterall, it's a city owned sidewalk, and I might get arrested, right?) Of course they'd say "don't step over the yellow tape, because it's a construction zone. etc... etc... etc....