Rust Problems

I have 2000 Mazda Protege that has a tremendous amount of rust on all four doors. In fact 6 months after I bought the car, I had to have the doors repainted. I took my car to 2 body shops recently and both guys I dealt with said that Mazda's rust very very fast. Why is that ? And is there any warranty for that kind of stuff? Tks all ! reply to group.