Presumably because she has an onward booking on the same PNR, and could not get out of the transit area without a passport.

I think the bigger issues, at least theoretically, are that (a) the immigration people in Guam would not know where she had come from, and (b) if anything happened along the way, she might be forced to enter Japan for lodging, medical treatment, etc.

BTW, this website seems to say that if you are in the military, you are allowed to transit NRT to get to GUM without a passport. Note that it seems dated (it refers to flying NW) but does note that civilians need passports: http://usmilitary.about.com/od/airfo...Andersen_2.htm