Its the "reasonably foreseeable" bit that counts here – a basic premise of negliegence
I see it like this:
Landowner has no idea what's built on his land, and someone falls off a badly built bit of shore or whatever. He's probably fine – he didn't know it was there, so the only route that could be argued is that he should inspect his land regularly, which isn't really going to stick.
However, landowner comes across a bit of badly built shore, and he leaves it, then there's a possibility that someone could do him for negligence if they fall off. He knew it was there and didn't do anything about it.
At the end of the day, the threat of litigation is as much an issue as successful litigation. The prospect of all the hassle of a court case is enough to make most organisations act very cautiously.