My understanding is that you should aim for the torso. It is easy to miss with a handgun, and people don't normally stand still when you are trying to shoot them. Unless they also have a gun - in which case again, you need to hit the target first, not necessarily kill. A shattered rib would normally be enough to upset their aim. If they are coming at you with a knife, you would probably have more luck getting a few shots into the chest than going for the head.

As for American gun culture, it stems from the constitution, which has taken on almost religious significance in American culture. Being granted a right in the constitution is better than being granted a right by God. I still haven't figured out why they have not altered that bit of the constitution. I suspect it is very savvy spin from the gun makers. It's hard to get through to someone who chants 'guns don't kill people, people do' as a substitute for rational argument. Plus, they have to deal with the Mexican border - which makes it a lot more true that the bad guys will still get guns than in a country like Australia that has no land borders. This is mostly down to economics - it is a lot easier to import guns into the US than Australia.

I don't think anyone knows for sure. They are still arguing about it a few hundred years later.