The airlines are in the business of air cargo. They move weight quickly from A to B. It doesn't matter if that cargo is alive or not, it still costs so many dollars per pound per mile for fuel to take that cargo aloft and fly it quickly to it's destination.
When I take a heavy package to the post office, they have the nerve to charge me more than they charge for a small package.
It's not my fault that my package is heavier than the others. Why should I have to pay more?
It is not the fault of the post office that it takes more fuel to fly my package overseas than it takes to fly a lighter package. Why should they have to lose money by charging me the same for a heavy package as they would for a lighter one? Should they raise the postal rates for everyone in order to cover the extra cost of sending my heavier package?
Up to this point, the airlines have taken a Socialist view on airfares. The lighter passengers have been paying a bit extra to cover the additional fuel cost of flying heavier passengers.
Why would it be unfair to charge by the pound?
If the ADA doesn't like it, let them pay the difference.