I understand, but given that the customer may not have known that he was purchasing a grey set, i.e. no deliberate decision was made to get a product not meant for the local market... While this can be conveyed, the refusal to service grey products could be seen as unreasonable. After all, in this case a camera is much more complex compared to say a CD. I can see why a local music distributor refuses to entertain customers who have purchased a grey market CD - after all, it's a CD, and it's not from me - end of story... But for the camera, you can look at the body, replace the parts that are causing the complaint that the customer has highlighted, and charge the time and parts (even with a small profit) to them.
I guess it really depends on what the local entity views itself as - if it's part of the global Nikon brand, then certainly a Nikon product from China shipped to Singapore as a grey product is still coming from Nikon the brand.. I don't think it's fair to argue that all grey sets are by necessity factory rejects. But of course, if the company's primary priority is to ensure that it controls the market and the sole source of Nikon products is itself, then that's another story altogether. By creating an unconducive environment for grey sets, it ensures that maximum profit from Nikon product sales is achieved as there will be reduced competition from external sources.