Looks to me like they are doing the land owner a favor...
IDK the situation on this post.
But, as a landowner, I would be pretty dang ticked to know folks on my property only authorized by law to be working were taking anything off of my land. One of the things I love is finding "junk", deer/animal skulls, etc. on my land.
It's not anyone's prerogative but mine to decide what is a neat find and what is "junk". Obviously, whoever is putting that stuff on the fence thinks it has a story to tell.
I have learned the hard way that many (if not most) people who have access to your land will pretty much do as they please if you are not with them.