I don't think you did anything wrong -- why do you think you did something wrong? Because they turned their heads when your ball landed? I've heard balls hit behind me, with no danger of hitting me (or even rolling to me), and I instinctively turn my head, maybe even look back, but it doesn't bother me as long as it's a rare occurrance. I know I'm sharing the course. You did nothing wrong.
I will play devils advocate here. However I will say I dont think the OP was truly out of line. 1. I was always taught that on a par 3, you wait for the golfers to clear the area (drive off in their carts). 2. Is it possible that 20 yards away is really closer to 10-15 yards? 3. Staying way back is many times worse than playing your normal game, because the group cannot tell you want to play through. 4. The ball landed with in 10 yards of them as you say, "Fore" probably should have been used.