Did the USA stop recognizing dual citizens? My daughter was told she is not recognized as a dual citizen?

She was told by a homeland security person (no badge number was taken & we don't care) at the airport as she was inbound from Europe. She would need a USA Pport to return to the USA next time. She comes here every few years. She is a citizen of this other country for over 10 years, graduated from college, and has a very good job. AND the country is NOT on a restricted travel list. I am looking it up on-line and plan to call INS after I get more info. My daughter left yesterday, and I would appreciate some added help.