Do keep in mind, as we discuss this unequal treatment of death, that much of what we are reacting too is stories from the news media. And those news sources are mostly for-profit institutions. Which means that they are defining what sells. The closer the dead, the better.
Of course that is not the only factor in mm's question. We Americans are indeed culturally brainwashed (via media and other sources) to value American lives more than others. Hence I probably react more vicerally to a disaster in Miami than I do in Vancouver, BC, which is far closer, physically.
Related to that is this: the American media to which we inhabitants are exposed to incredibly prejudiced for America and ignores the rest of the world in general. Any foreign visitor to the US will tell you how shocked they are about the dearth of news we are given about the rest of the world, if we rely on American sources. I certainly became more aware of the global situation with the advent of the Internet and the sudden availabliity of the Guardian and the Paris Match and the Jerusalem Post and The Times of India, sources I had never had before. Not that I read them daily, but at least now when I hear of a major calamity elsewhere, at least I can find sources that aren't quite as interested in addicting me to Pepsi.
I can't speak for others, but I am not immune to tragedy elsewhere. When US drones killed 60+ schoolchildren in one attack, I was stunned and shocked. However, I have to admit the killings in Connecticut stunned and shocked me more. Which means that not only do I have prejudices, but that I see a difference between military-caused deaths and those caused by madmen. When there isn't always such a difference.
Of course we are also always up against this reality: How much can one person take in, shock-wise. I don't think that many could react with equal shock/horror/disgust at the news of a local disaster and one halfway around the world that were otherwise equal in scope. An earthquake killing 100 people where I live would certainly affect me differently than an earthquake killing 100 in Iran. I do not have enough cellular matter in my body to call them both equal and survive.
One more factor. The aforementioned money-grubbing media reports disasters that 50 years ago would go practically unnoticed. In the sixties, I was exposed to 15 minute news programs on TV and one local newspaper. And there was nobody to teach me that everybody mattered. And since we won our revolution, I was not part of the world-wide empire of a European nation.
mm's question is a good one. We should all ask it. But the blame for our shortcomings has a wide range of reasons. Some need to be addressed personally, such as prejudices. Others socially, such as suck-ass news sources.