Originally Posted by
uanj
Korean carriers are not allowed to transport passengers originating in the US 50 to GUM transiting through ICN. However US carriers are allowed and as an example UA does route passengers from the US 50 through NRT to GUM. What happens if one leg is a US domestic carrier and one leg a foreign carrier? Not 100% sure but like you I bet UA knows and would not issue a ticket if not allowed.
Having lived on Saipan (SPN) and followed the Asiana imbroglio closely, I can assure you that if one leg of a GUM/SPN trip to the mainland U.S. via ICN/NRT, etc. was on a U.S. carrier, that would completely cure any cabotage issue.
There is a comprehensive FlyerTalk thread in this UA subforum on cabotage.