OMEGA has an international warranty so If you buy an OMEGA watch from an authorized dealer with stamped cards in any country in the world then it is applicable anywhere no questions asked.Generally they may come from overseas markets and will not have a valid US warranty.
Rather like why can I not watch a video or buy a book from Europe? Sometimes the distributor does not authorize the sale outside of that specific market. Go figure.
ADs are not wholesalers and they can't export watches to other countries, but once watches are sold to individuals they have no control nor they are required to follow up on them.