I would like to know how they're going to prove you were the cause of the tranny going bad.
I used to work in a Nissan service department and learned two important lessons that apply here.
1st, Service advisors are under a lot of pressure to avoid warranty work. It doesn't pay nearly as much as straight time. That means the dealership doesn't make as much money, the service advisor doesn't make as much money, and the tech really doesn't make as much money. Actually, techs get screwed HARD on warranty work.
2nd, anything is possible. This is good and bad. It means that if you don't get an answer you like from the advisor... demand to speak with the service manager. If that doesn't resolve it, let him know you're unhappy and that you want the 1-800 Nissan complaint #. (Service departments are rated by Nissan, most managers care a LOT about this as it's usually part of their bonus structure). It's hard, but demand what you want (a free tranny). Stay constant to the "I didn't abuse this tranny, I drive it normally, I am very upset about how I'm being treated, I want this resolved and I'm not going to accept anything less" theme.
I once worked on a lease vehicle that was comming in on trade in. It was only a year old, had 40k on it, and had never had an oil change! The engine was screwed... rod knock, oil like tar... I litterally had to wait well over an hour for all the oil to drain (it came out in clumps). Guess what, they took her vehicle in on trade in without any penalties... because she bought a new car. Anything is possible... it all depends on what they want to do. Make them want to give you a tranny if you start to get resistance.
Stick to your guns and you'll get what you want in the end. If all else fails... there's always other Nissan dealerships. If the dealership absolutly won't give you a free tranny, go to a different one.