Originally Posted by
gardener
The overweight baggage fees are meant as a deterrent, so actually it isn't "whatever the market will bear", the airlines try to set it higher than that.
Reason is baggage handlers can be injured lifting heavy bags - and that costs the airline many times the extra fee.
This is my thought also. The fees is generally greater than the incremental cost of carrying the extra weight, but the goal isn't to pay for the extra weight, it's to prevent it.