Earlier quoted context omitted.
Yes, it does suck. You are taking "locals" a bit too literally. It is just a social convention and these are small communities, it is generally understood that unless the property owner has socialized something different, those signs are for mining companies, yahoos from the city, etc. A lot of times if you are not from the area, you can often ask one of the local people the purpose of a sign in some area based on wh…
I'd share your frustrating with mining companies, but is "yahoos from the city" a bit unfair? Your country has brilliant appreciation of the outdoors, including city-dwellers who travel out on weekends or holidays. Signage that informs and guides them rather than bars them might help. I imagine 99% of people just want to pass through carefully rather than dump trash or graffiti anything.
More often than not, it wasn't that people were trying to behave badly, they were just careless and exercised poor judgement. A recurring annoyance was someone in the Bay Area buying an SUV and deciding to take it off-roading deep into the wilderness with no prior experience and being completely unprepared. In addition to frequently damaging their vehicle beyond drivability or getting it hopelessly stuck far out of mobile phone range, they would tear up the roads and land joyriding, leave gates open, run into the cattle that live in the wilderness, etc. I probably personally rescued people a dozen times who had no business being out there in those vehicles (an SUV off the new car lot is not equipped for that environment), with that lack of preparation, with that little experience.
Not malicious most times, just ignorant and engaging in poor behavior without understanding that they shouldn't be doing those things.