Originally Posted by ATOBTTR

The fee is there to discourage overweight luggage in the first place. Two main reasons are for the safety of the ground crew loading bags (heavier bags increase the chance of injury) and for weight and balance reasons on the aircraft. Two bags at 50 lbs are easier to balance across the available space of the aircraft versus one 100 lb bag. The issue becomes even bigger on smaller jets like RJs with a single cargo bin.