Front Suspension Control Arm problems of the 2002 Chevrolet Tracker

Received a letter from gm stating that my 2002 chevy Tracker might have a corrosion problem with the front suspension cross member because it did not receive adequate corrosion protection. Corrosion may progress over time therefore causing the front lower control arm bracket to separate from the crossmember. I brought my car for the free inspection and found out my car did indeed have this problem. The letter states that the crossmember will be replaced at no charge. I was told by the shop manager that this would cost me between $1200 to $1500 and that gm will only cover the cost of the part ! I am puzzled??? why should I have to pay any part of this ??? should gm be responsible for the entire bill ???.