Originally Posted by Steve M

One thing that fails to get mentioned when discussing US or Canada immigration questions is that the immigration agent is also doing a Customs mini-interview. Consider that for returning citizens to their own country, there's really nothing that needs to be asked from an immigration standpoint: once the traveler's identity and citizenship has been established by the passport, they have an unconditional right to be admitted to their own country (at least in the US and Canada). There's really nothing else that needs to be asked in order for admission to occur. The questions are really part of the Customs process, where at an airport the immigration officer might mark the customs form with an indication that the traveler be given special scrutiny by a Customs officer for some reason or another (or be sent inside in the case of a crossing by car).

This is different from other countries, where the duties are strictly separated: the immigration agent will ask questions relevant only to immigration matters, and likewise for customs officers.