Air shippers offer a money-back guarantee on timely delivery of the baggage. If an express cargo package is bumped from a plane, the very next plane will take it to its destination, because failure is expensive.

My checked baggage, on the other hand, is a strictly best-effort affair. If it doesn't fit on my plane, it becomes a "gets there when it gets there" problem. Once it's bumped once, the airline sees little reward in getting it to me faster or slower. There's good odds I will never see it again.

Comparing a $25 checked bag fee to a contracted cargo shipment is quite unfair.

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edit: Here's another fun item. Technically, the conditions of carriage on your ticket don't require them to bump cargo for your baggage. And there's no incentive for them to do so.

http://www.bloomberg.com/bw/articles/2014-07-09/a-lawsuit-co...

“It costs United nothing to remove checked baggage, rather than cargo, in order to arrive at an acceptable aircraft weight because the checked baggage fee is purportedly nonrefundable, and United can simply transport the checked baggage on a later flight to the passenger’s destination.”