I agree with you, but I believe Ford is looking at it from a money standpoint, unforetunately.
They way I understand it, if Ford chooses a voluntarily recall, Ford gets to decide how to execute it. If the NHTSA executes the recall, than Ford is at the NHTSA's mercy as to how the recall is executed. Ford voluntarily decided to recall year 2000 vehicles (certains makes), so Ford saved themselves some money by limiting the recalls. NOW, the NHTSA is conducting an investigation which could end up meaning more recalls (and more money for Ford). I'm sure Ford is trying to come up with a way to prevent the loss of money, somehow. Maybe Ford can do another voluntary recall to save their necks (say, extend the recall to 2001 vehicles - for example). Though, I don't know if they're allowed now that the NHTSA is involved.
I think Ford should replace all vehicles that carry the same or similarly designed defective part, but I'm sure Ford's financial people would probably disagree with me.