Originally Posted by
rubesl
The U.S. does not have exit controls - showing your passport (Korean) to the airline is so they know you can legally enter the country you are going to, otherwise the airline can be fined if you are refused entry. If you are a U.S. passport holder, even if you hold dual citizenship, you MUST enter the U.S. on your U.S. passport.
That has been my experience as well. The U.S. does not forbid dual citizenship and the only rule I know of is to use one's U.S. passport when entering the U.S.
I even know of a person who entered at a land crossing using the foreign passport, explaining that the U.S. passport had been accidentally left at home, and was waved through. I'm guessing that would probably be up to the individual discretion of the agent, however.