The US is the only industrialized nation that taxes its citizens who live in other countries. The only other country that practices citizenship-based taxation is Eritrea. Many US citizens who live in other countries must pay income tax to the United States as well as to the countries where they reside.
More about citizenship-based taxation:
Due to variations in currency exchange rates, citizenship based taxation results in US citizens having "phantom income". It's not real income. In fact, it can even be a loss, but because the value of the US dollar varies in comparison to the US citizen's working currency, it can look like income exists when there was no income.
In these cases, the IRS wants taxes paid on this phantom income, and there are no foreign taxes to credit against it.