I think of this situation and I think of the 7th hole at Bethpage Black. If you hit your drive in the far right end of the fairway bunker (easily done if it's into the wind), you're about as dead as it's possible to be. The trees on the green side of that bunker are huge and right there. You can't go through them (reliably anyway). Pretty much your only shot is to play out to the left of them, away from the hole.
But, if you want to get your tee shot into a spot where you can reach the green, you have to hit it on that line. You can avoid it and you'll be likely in the fairway, but 280-300 yards away from the hole, from whence I at least can't get there. I actually think it's a pretty good use of the double penalty. The slope of shots to hole out by distance is pretty steep in the range of where your ball will finish after that tee shot (so 10 yards closer to the hole from 250 to 240 is worth more shots than from 150 to 140 for example. So the benefit of taking on that carry is higher. But the penalty is severe. It kind of has to be. If the trees weren't there, then there'd be no reason not to go that way, since from the bunker you'd be laying up to a similar spot to where you'd be hitting it to if you played it safely. Similarly if it was just trees and no bunker, then again you'd play that way. Getting through the trees from grass is doable (you can hit it low - can't do that from the bunker, because the lip necessitates hitting it up).
Tom - I haven't played Sebonack, so I'm not familiar with the hole you describe, but I wonder if Jack was thinking of a bunker with a lip that makes you hit it higher into the branches of the tree, where from the open sand area, you can drill one under the tree limbs. I could see that being a difference. It does sound to me like he's thinking like a player rather than an architect there though. If I'm right, he's thinking that he should be able to have a shot at the green from there. A less generous person than myself might say he shouldn't hit it in there in the first place. That said, most anyone can play a shot from the bunker to the fairway. Hitting it on the green from the bunker (or avoiding the tree from a waste area) is what separates. He probably wants to be able to separate himself from the crowd.