Did you read the licence agreement that comes with iTunes and your iTunes purchases? If I am not mistaken, when one buys from a US app store (or any country specific app store), then the licence is not world-wide. In effect you have not bought a song, but the rights to store and listen to the song in a specific country. Thus, you might find that there is very little you can do to remedy the situation. Of course, if others have information to the contrary feel free to comment....
If it is true that purchases in one country's iTunes store are not transferable, then like you I would hope that Apple would have some reasonable way of recognizing that you have already purchased items in another country. I suspect though, that Apple has to pay by country, so if they did this they would have to go out of pocket (don't blame Apple blame the profligate greed of the entertainment industry...). Also, I assume that Apple would have to have a pretty sure-fire way of confirming who you are, lest you be some sort of hacker trying to engage in identity theft.