While I sympathize, remember what a warranty is. When a company sells you a car, included is a warranty of merchantability and a warranty of fitness for a particular purpose. Essentially, it means the company is responsible for ensuring the car you bought performs as it should. However, in every contract I write (not automotive), I make sure that I only warrant to the extent the product is appropriately used.
If you aftermarket your car to rag it out, potentially causing or accelerating wear and tear, I can understand why the dealership would want to void the warranty.
FYI, this is the first time I've EVER taken the side of a dealership. I feel kind of dirty, and not in a good way.