To be a bit more specific, it's charging citizens tax on money they earn from overseas when they live overseas i.e. nothing to do with the US at all. Your comment could be interpreted to mean just taxing income from US investments while living abroad, for example (which would be more reasonable, but still over-reaching). Taxing worldwide income is also done by a few other countries but only the IRS has the bizarre extra reach into overseas banking data (that I know of, at least).