Frame Rust problems of the 2009 Nissan Cube

I have a 2009 Nissan Cube. The Cube was at the dealership for maintaince. I was informed that the Cube has a exhaust leak, was told it is not covered under the emission warranty. I asked more questions about the issue, an was told by the dealership that it is a rust issue and not covered. I was told that they do not have the part. I again asked about the emission warranty, was told it did not apply. According to the warranty which is cover up to 80k miles for emission issues, but because of "rust" it is not covered. I do not agree with this and I call the 1 800 number and reported it. Today the next day I was contacted and persson said, or agreed with the dealership. I looked up complaints and recalls, there are several complaints. There is one in which it referrs to a "corroded" part. What is the difference between a part that is "corroded" part that may fail and a part that has rust and broken causing a exhaust leek. They Nissan and the dealership are using a play on words to avoid making the repairs under the emission warranty. I was told it is okay to drive and polute the enviroment since they will not cover it and they did not have the parts. After speaking with the rep from Nissan he had a whole lot more info about the repair than was listed on my copy of the repair order. There are four major componets of a exhaust system on most cars. The manifold, the down pipe, the mid pipe and the muffler. The "corroded part" or rust caused one the conection to leek. Now I am being told that 2 parts need to be replaced and the cost is over 1k, which I have no infor on my invoice or repair order about. The only recomendation was 'ehaust leak 395. 00" I know that dealships replace parts that have rust and or are corroded because I work for one, frames and rusted bolts. How can one replace parts and the other not.