It's their land, their choice what to do with it.

I maintain the ones on my property since I use it, some neighbors might, and I'm fine with the public using it. Someone started making their own bike trail through a sensative area, so I made those signs go away (the orange flags they put up) and they took the hint to use the established trail a short distance away. Other than that we put up "no littering" signs and people stopped tossing dog poop bags and trash so people are generally good.