The contract says so. ASUS was not involved in any kind of direct decision making rejecting them warranty, even if they opened the ticket and did a QA, the final decision and arguments are made by the service location and they have the right to argue. It is totally their responsibility including to prove that the damages caused by GN are connected with the claimed problems. I can actually bet there are as they were confident to reject the warranty, usually service locations are very careful, if not, they got a monkey level service, were extremely unlucky, it happens. Techs get old, die, students come in, knowing nothing etc.
I don't even know why people tend to glue on the idea that warranty is so tied to the maker. RMA is totally different world, for the most part of the world even the maker doesn't cover the costs, but the distributor, it was bought on special terms, warranty periods etc, it can be so complicated sometimes who owes something and pays for the repair. They were so hasty wiggling those papers, those are only papers with some ASUS logos on those also.