OK, I stand corrected then. I remember reading another case where they did not expect the USC to leave, but maybe that is because of different circumstances. So it looks like they decide on a case-by-case basis according to the background of the USC and the nature of the foreign country. I doubt they would expect a born USC who never left the US and has no family in Iran to move there, whereas they would think differently for a naturalized US citizen who grew up in Ireland, as far as moving back to Ireland or the UK is concerned.
I suppose they also have to consider whether the USC would even be allowed to move to their spouse's country in the first place. The other country may have immigration restrictions that would prevent that, especially if the USC has a disease like HIV or a criminal record.