I believe this is all part of reciprocation between the US and other countries. If your passport is no longer valid when you try to board to return to the US, they won't let you on the transport (ship, plane, etc.). Consequently, the country of departure has to have sufficient assurance that when its time for you to leave, you will be able to. I'm not sure of the Jamaican visa requirements, but it may be something like 6 months... I can't remember the JA I&C document that you have to fill out prior to entry, but I know they ask how long you will be in country.