Landowners own the land, but they can decide to not prosecute people "trespassing" over the corners (which, AFAIK, has no effect on their private land in any way whatsoever).

I'm sure most landowners aren't POS and most do not. But this guy did, and gave the hunters a big fine and a big legal challenge because they were in his "airspace" for no more than a couple minutes. Which is why we need a codified law, so hunters and campers don't even have to worry about this in the future.