Rear Window Defogger problems of the 2010 Mazda MAZDA3

Rear glass explosion on 2010 Mazda 3 sedan. Morning 16 degrees f. Approached my car in the morning and heard a crackling noise. I thought the sound was ice. I started the car and the rear defogger. As I sat in my driveway . . . Approximately 30 seconds after turning on the defogger. The rear glass exploded with a load noise. Glass was thrown into the rear passenger seats. The car is 4 months old and under warranty. I took the car to the dealer and they would not repair under warranty. I called Mazda customer care at 1-800-222-5500 and was told that this is due to outside influence (ie the same as if a tree branch fell on the car). He indicated that I should expect that this could happen in the cold weather. I can not believe that I should expect that my rear window can explode in cold weather. If my children were in the rear seat they would have been potentially seriously injured by thrown glass.