If you spend enough time looking up how to travel with multiple passports on the internet, you'll invariably run across hundreds posts telling you that, by law, "US citizens must enter and exit the US on their US passport." I am also very aware of what 8 US Code §1185 says, which is as follows:
Except as otherwise provided by the President and subject to such limitations and exceptions as the President may authorize and prescribe, it shall be unlawful for any citizen of the United States to depart from or enter, or attempt to depart from or enter, the United States unless he bears a valid United States passport.
I fully understand what it means to 'enter the US on a US passport' and I can't figure out why a US citizen would ever try to enter the country on another country's passport anyway (if only to avoid the even longer lines at the border).
But what does it mean to 'depart the US on a US passport'?
The US obviously doesn't have standard exit immigration controls, like you get in nearly every other country (save the UK, Canada, Ireland, NZ, and Australia). So how can you 'exit the US on a US passport' in the first place?
Does this refer to what passport information you give to the airlines?
Obviously airlines are not border patrol, and I know the airlines are primarily concerned with whether you have the permission to enter the country you're going to so they can avoid getting fined by countries if you get denied entry. But they do also share your biographical data and passport info with the countries that you're exiting and entering (e.g., APIS in the US). The US, in particular, also uses this info to generate departure information for non-citizens' I-94s, so one could make an argument that this does constitute 'exiting the US'.
If it does refer to what info you give the airlines, then would people giving the airlines their non-US passport details (either for visa issues or just out of convenience) to for a flight out of the country technically be breaking the law?
If it doesn't, then what is this 'exit' stuff that the law speaks of? Is it just there in case the US decides it wants to start doing exit checks? I know that on very rare occasions, CBP does do spot passport checks at gates for flights departing the US, but this is far from the norm. And in such a situation, I wouldn't even think of ever showing a non-US passport to CBP as a US citizen.
So, fine people of the internet, what think you? Is this wording just some legal nonsense, or does it have practical significance?