I'm not faulting her for missing. Obviously using a rifle that hadn't had its scope jarred out of alignment would be a plus, but that wasn't her fault, and she might have missed even with a sighted-in rifle.
Unusual, unexpected events happen. The four rules were not handed down by God, but they exist so that in the worst case, not just in the average case, nobody gets hurt. In the worst case, Palin might have hit someone, picked up a negligent homicide conviction, and would have been finished as the right-wing's media darling. If that had happened, would it have been worth it just to get a clip of a caribou silhouetted against the sky as she took several shots?
How many people do you think watched that episode and didn't consider the backstop issue? The risk that she might have killed someone is entirely Palin's problem, but beyond that it sets a bad example.
There are some places where there is NOBODY for a LONG, LONG WAYS
There is PROBABLY nobody for miles, but that doesn't change the fact that if there happens to be someone out there, and you hit them, you've committed a crime, and that's what the backstop awareness rule is designed to prevent.
It wouldn't even have to be a human out there. What if she'd hit and wounded another animal, leaving it in agony for hours or days before it died? And she probably wouldn't have even known. For all we know, that could have happened.