Like you say, there is acceptable risk, and the people who were in charge decided it was impossible to cut off the attack. I have heard reports that two americans serving in the area did try to attack the attackers. And they ended up dead too. There was just no proper way to counter the attack, and the attackers were obviously prepared for some kind of attack.
The people in charge decided the election was more important than American lives. They also knew they had the MSM in the bag for them who would convince morons like you the administration did nothing wrong, and try and bury the story.
It's why we know more about the Romney's 47% comment than we do about Libya.