Isn't the most commonly referred to law that asserts a requirement for US citizens to enter the US on a US passport also the very law that asserts a requirement for US citizens to exit the US on a US passport? Yes.
http://www.law.cornell.edu/uscode/text/8/1185
The applicability and power of that often referred to law is rather limited, although neither DHS nor State are willing to highlight the full extent of how its applicability and power is limited under law; its applicability and power is more a function of extralegal, administrative action taken by DHS than of DOJ action in court against a US citizen entering or exiting the country with or without a US passport unless the US citizen has been prohibited by court order or some other law.
Originally Posted by
Fredd
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.
The US does have exit controls, however, they don't operate in the way they do in many other parts of the world with physical, fixed exit passport control checkpoints prior to exiting the country (at least by air on a common carrier). We do have electronic exit control checks to a rather large extent when it comes to flights out of the US on common carriers; we also have some gate-area checks for various purposes.
Originally Posted by
Fredd
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.
The CBP employees have no legal right to deny admission to a person who is recognized as a US citizen, passport or not; however CBP employees do exercise discretion on whether or not to send such a recognized US citizen, passport or not, to secondary to validate identity and/or citizenship or to otherwise hassle the person/party for complicating matters and/or being in a somewhat more-than-usual complicated situation.