Most intenational travelers check luggage. NW flights must have "luggage match" to fly international (and even domestic it seems) That means your butt must be in a seat on the same plane that your luggage is on. I think it is one of the myriad of rules imposed after 9-11. I have been told numerous times flying on NW metal INTl that since I checked luggage I could not volunteer. Have even been told it a few times domestically trying to stand-by for an earlier flight, due to the luggage match rule.
I think that would have more to do with it, than his connection, or routing. They generally do not want to hold the flight in order to get a volunteers luggage off the plane and delay everyone else.
However, flying US Air last week, I was bumped off the flight and put on an AA flight to DFW and my luggage stayed on the US Air flight. So it does not seem like a fool proof rule. Of Course with the situation with Luggage and US Air last week, I think they would have done just about anything to get luggage to the final destinations.
My opinion, is that the agent was following policy. I could be wrong, but that has been my experience.