Many US territories have their own customs laws/areas and some also have seperate immigration rules. I've always needed a passport for American Samoa and they process CBP in HNL on the way back. My trips to Guam have customs checks, and they have their own Visa Waiver program. CNMI are different from that even (or were a couple years ago).Maybe there might be some wierd ID/Birth cert loophole but everyone on my flights has passports.
IIRC the VI has their own customs rules, can someone confirm?
Also IIRC Most territories are unincorporated and thus not part of the 50 states. D.C is incorporated.