My guess is the park areas is the original location of gold discovery
?
Dozer-bitch, there are scores of places in scores of cities, with loads of history. Where you will not find a muni code like that. For example, I live in the 2nd oldest European toe-hold in CA : Monterey . Second only to San Diego by one year, 1769 vs 1770. Yet there isn't any *specific* verbiage that says "no md'ing" in our parks here.
OH SURE someone can say "scram" if you're digging in nice manicured turf in front of nosey-parkers. Or if I waltzed into one of the couple of *obvious historic sensitive* monuments here. But no, there's nothing specific. Just the ancillary stuff (alter deface, cultural heritage, Lost & Found, harvest & remove etc....)
that exists anywhere. (to address persons who are being a total nuisance)
So to answer your question of why some lone isolated cities ever dreamed up something specific, it is because md'rs in decades past (not you, but others), went in swatting hornet's nests . Asking "Can I ?" questions. Then their "pressing question" gets bandied around, desk to desk, till it finally gets the "safe answer" (from someone who envisioned geeks with shovels). Presto, another law or rule or "no" is born.
I started this hobby as a young teen in the mid 1970s, and saw the evolution of most every law & rule that can, yes, be traced back to this origin. NOW OF COURSE the rule might allude to "d/t cultural heritage" or "d/t holes". So the average md'r says "durned that md'r that must've left holes" or "durned those archies". But the bigger question is : WHAT PUT IT ON THEIR PLATE , TO NEED TO ADDRESS, in the first place ? So their 'holes" and "heritage" stuff is only their "go to" rationale. For if someone ever asks
"but why?"
It is a giant case of : "No one cared till you asked". A giant self-fulfilling vicious loop