Originally Posted by
corporate-wage-slave
That will work for one or both of two reasons: as a US citizen you are able to enter the country concerned and are in good standing with those countries; secondly these are countries that don't process API information. You have to be a bit more careful with some other locations, Australia is one that comes to mind. You are of course required to use your US passport on travel to the USA.
I entered Australia on my UK passport, obviously with a visa. Now I do not remember anymore if I changed it in API, could well be.
Oh well, I do sometimes run into issues when the AA system seems to think I need a visa to enter the UK. I then just show them noth passports and all is well. Would be convenient if BA would allow to enter 2 or more passport details and that you can then select the one you wish to have connected to the specific trip.