Chris. Good points that I can tell you are thinking through fairly.
... the official wording of the law already calls it criminal trespass to enter a property and use it for other than its designated use and It is no defense to a charge under this section that the land or premises involved was owned, controlled, or in custody of a public agency.......
I'm sure this can be said of any speck of public land. It's the nebulous catch-all ability the governing authority can use to boot persons for using an otherwise public place, for PURPOSES FOR WHICH IT'S NOT INTENDED. I get it.
There is always catch-all verbiage such as that, like laws that "forbid annoyances", for example. Because it's simply impossible for laws to be written to cover every conceivable thing that *might* come up in the field. Hence these catch-all type wordings exist. To employ as need be, too usher away someone being a nusiance.
But me thinks your taking this and turning it around the other way. Such that an activity needs to be *expressly allowed*, in order to be able to be the "purpose for which it's intended". But think of it, when have you EVER seen a park, or field, or sandbox, or beach, that was labled "for metal detecting". Or a sign saying "metal detecting allowed here" ? See ? There is never any land set aside for the express purpose of md'ing.
So I do not see the statute you list as necessarily precluding md'ing. There are scores of things that go on in parks all the time: Bird-watching, frisbee, hopscotch, etc... Why does one need to assume that md'ing is disallowed, till given a princely express "yes" to the contrary ? Any more so than other innocuous activities ?
Yes I agree that someone in authority does indeed have the say so to come alert you that they feel your chosen activity is outside the scope of "purpose". In the same way they could come tell the person flying the frisbee to stop (heck, you might poke someone's eye out). But does that mean you needed to pre-empt it with permission ? No.
... A court house lawn is not at all the same as the parks ,....
Perhaps it's not a county or city "park". But haven't you noticed how they are defacto parks anyhow ? Eg.: park benches, trees, lawns, drink fountain, blah blah blah. In every sense of the word park. Whether or not it's merely an expanded parking strip that just happens to surround the court-house, or whether or not it's listed in their list of "county parks", is a non-issue. Because the bottom line is:
a) the public is allowed there.
b) it functions as a park (all semantics aside)
c) the rules of use (ie.: things forbidden) are no secret. Anyone can look them up in county ordinances, and/or park rules (both if they're skittish and want to cover both bases fearing it's not a "park").