Many citizens of foreign countries also happen to hold a U.S. citizenship without ever residing in the U.S (or only doing so as children). They may have it because they were born in the U.S. or received it from their parents.
Regardless of how the US citizenship was obtained, they are still required to file an annual tax return with the IRS on their worldwide income. Same as all U.S. citizens who live in America.
Unfortunately many do not realize this - until one day when they discover that they are many years overdue and need to fix it ASAP. Most common occurrence is when their local bank asks them to sign Form W9 - indicating that they are a US citizen.
We have helped hundreds of dual citizens normalize their status with the IRS and are familiar with how to best execute this process.