This $15 bag fee is a scam anyway. The airlines aren’t going to eliminate enough weight to make much of a fuel difference. If they want to save money then the best ways to accomplish it without punishing the flying public is more direct routes and slow the planes down.
I am not certain if that is the reason. My first thought was they were trying to eliminate baggage handler's. Recognize that if a baggage handler makes 40K a year the elimination of the position probably saves the airlines at least double that figure in direct and indirect costs.
This doesn't seem to be a money saving but a revenue generating issue. At $15, each bag pays for a little over gallons of jet fuel. 100 passengers paying this could pay for the fuel costs for a part of the flight.