I don't know how common this kind of limitation for the warranty, but I had the same kind of experience with a bag repair before. A bag maker only offered a one year warranty on bags which were purchased at their online and physical stores and at the authorized stores like department stores. They didn't do any repair work if bags were sold at any other places. It was not written in the bag maker's website. They are internationally operating company and they had the same repair policy worldwide.
I understand your frustration, but as other members already mentioned, that's their way to minimize a chance dealing with possible inauthentic products. Even a small size tooth paste sold at 99 cents stores is faked, fakes are everywhere and it's getting easier and easier to make fakes.