The spectators are ultimately responsible. But the tours can do a better job of managing certain situations where spectators gather and want to be up close to the player.
I'll never forget my experience following Arnie back in 2000 at the local Senior US Open. He was roughly 71 years of age, no longer competitive obviously... it was a ceremonial appearance. So I'm just roving around the course and see that he's on the tee of this medium length par4. There were throngs of people around the tee box, no chance to see him up close. So I race down to the fairway hoping to find a spot... a safe spot. I was standing under a tree. Arnie was the last to tee off. I saw three balls land in the fairway 30 yards ahead of where I'm situated. Then I hear "fore right!" Arnie's tee ball clamored around in the tree I was standing under and dropped 10 feet away from me.
5 minutes later Arnie's there at his ball. He's blocked out from a straight shot to the green, lying in thick rough. He grabs a 3wood from his bag, needing to hit a low cutter to the green to avoid hitting the tree.
He's standing over the ball, trying to figure out how he needs to hit the ball to get it to the green. He sees a spectator standing very close just 10 yards ahead of him. He says, "Hey, fella... I just missed a 40-yard wide fairway and my ball was on a peg. It would be wise to move a little further back. Just in case."
The gallery around him chuckled, he smiled and chuckled too. But he was also being serious.
These tours aren't liable when things like this happen, but I think the tours could also do a better job shepherding the galleries out of immediate dangerous situations. I see some of these wayward tee shots on tv, everybody's got their cellphones out and taking pictures and video, with absolutely no awareness that the smallest tuft of grass could suddenly alter the face angle and hurl a projectile at them at over 100 mph.
I'm surprised more fans aren't seriously injured. These guys are the best in the world swinging a club, but they're also prone to error. Otherwise they wouldn't be trying to hit low lining bullets from the trees and hitting through a very small gap. I see how close some of them just 10 yards ahead stand and just think to myself - there's no way I would stand there where they're standing.