[Burgers]

I'm not claiming the work becomes safer if you charge more but the charge can help mitigate some costs should a worker become injured - onerous penalties discourage those that want to carry a heavy bag - but in the end the customer has a choice.
I tend to think its all about health and safety, here's why. You could pack your 10 extra pounds in a trash bag and check it for $25 or pay the $90 for a single 60 lb bag. Clearly they are discouraging heavy bags.
Watching workers handle bags in cramped baggage holds on their knees, or twisting and lifting as the put bags on a belt loader I suspect airlines have a lot of OJI claims.
While I would agree a maximum weight limit makes a lot of sense in the US we don't have such ridge rules - there is much more to consumer choice than say Europe.
Benefits costs are particularly onerous on companies in a tight margin business such as the airline industry.