Why is four years the limit for the repair to be covered?

The video card in my Jan 08 MacBook Pro suddenly went out and I believe it is one of these faulty units but it has been 4 years and 3 months since I bought it. So now, according to the Genius, I have to pay to get it fixed BUT I get a discounted rate (300 instead of 600) because I've kept my computer is such good shape over the years. How is this fare? If it is a defective unit shouldn't Apple replace all of them no matter how long it takes them to fail?