As with many things in golf, much depends upon the situation. Can trees prevent a ball from travelling further? Sure. Can the prevent all balls? Depends. Variables like elevation, topography, species and size (a 40' Spruce is denser than a 40' Oak) and density all need to taken into account. Unfortunately one usually ends up with a Damned if you do, Damned if you don't scenerio. On a tight course that had trees but they were removed (or died) the average person would ASSUME that they would have protected adjacent holes/players.
I find it amzing that our legal system has evolved to the point where the injured party is usually found to be less than the magical 50% responsible. If the case cited, here you have a guy on #18, which means he already played #1 (and being a muni, probably played that course numerous times). I wonder how many times he either hit, or witnessed a playing partner hit, a ball into the rough between the 2 holes? and then to get hit in the eye, the thing you see out of. That means he was looking towards the 1st tee. Maybe he should change his name to Homer Simpson.
Tom, I heard that story about #1/9 at Pasa. It was conveyed to me that originally Mackenzie wanted St A's combo fairway for the 2 holes. Granted #1 tee is elevated. Since I have a bad tendency to go left off #1 tees, I have played both 1st holes at St A and North Berwick from the 18th side of the fairway. So, should they both be made to plant trees to protect golfer from me?
Actually at Pasa, the OB left forced me to aim further right than normal and guess what? Yep! The dreaded straight ball. I found myself in those trees and blocked from advancing on the 1st hole unless I played up 9. So, there is a situation where trees planted specifically to protect golfers actually were a cause for putting me in harms way (especially with how narrow 9 felt with the houses OB left.) Beware of the Law of Unintended Consequences!