So far, the discussion has ignored any foreign income taxes on Sue's surgeries. It now is time to look at this double taxation –- US income taxes plus foreign income taxes.
An obvious preliminary question is why do foreign income taxes, as compared to other costs of doing business overseas (like other foreign taxes), present a special case? The idea is that most costs are reflected in the prices at which goods and services are sold. Federal-level income taxes, however, are bone by the owners of the business. Thus, being required to pay US and foreign income taxes is double taxation. (The born-by-owners assumption is shaky, but further analysis is beyond this little post.)