There are three types of people in the world, those with more than an ounce of common sense that don't need told the glaringly obvious, and those with almost no common sense that do need told but take no notice anyway. Thirdly, and I guess the sign is aimed at them, there are people with no common sense whatsoever. These are the folk who would walk through the woods when it wasn't safe to do so, they'd get hit by a tree and then look around for someone to blame, because "nobody told us that trees can fall down".
I remember when I was in junior school and I fell off a tree swing in my mates back garden. I ran home screaming the place in because there was blood running off my elbow. Ten minutes later Mam had spat on a hanky to clean the wound, stuck on a plaster and sent me back out to carry on playing on the same swing. The point is, my Mam didn't go round and beat his Mam up, there was no long legal battle while we tried to get compensation, tree swings weren't banned if you were under ten years old and I doubt that any warning signs were put there to warn everyone of the dangers.
Then the 21st century arrived and we now see signs like this one pointing out, as I said above "the glaringly obvious". How times have changed.