Citizenship-based taxation is rare, and currently the US is the only country to tax citizens who live abroad on their worldwide income. This means that US citizens are forced to report all of their income to the IRS and are subject to US taxes on it regardless of where they live and what they do.
The reporting burden alone is difficult and often expensive because it is too complex to do without professional help, to say nothing of the double taxation that can result. Furthermore, it hinders a US citizen's ability to move and work throughout the world because it can place a further burden on companies who hire US citizens abroad.
Please follow the practice of other modern, developed nations and tax US citizens abroad in the same manner as nonresident aliens if they pay tax elsewhere.