My mother was almost denied entry into the US because her passport only had a month left on it, and she was only catching a connecting flight through the country to return home to Canada. She was told they impose the six month rule because they want to be sure that if anything prevents your departure (illness, accident) that your passport will still be valid when you are ready to return home. Also, that some people trying to avoid the immigration process will arrive, let their passport expire and then make the claim they can't leave.