Well, as far as sculpture being subtractive. Say you are whittling a piece of wood into a boat. The form of the boat already exists within that piece of wood and you are removing or subtracting physical material in order to expose it.
I believe a photograph is extractive because you are focusing in on one area of a whole and extracting just that one square/rectangle. You are not physically subtracting what's outside of that square but choosing NOT to extract it.
This is not my idea, I have also read this somewhere but I'm not sure of the source. It might have been "Art & Fear" by David Bayles and Ted Orland...