>your overly simplified logic "You should pay more because it costs more money to fly you" obviously makes no sense.

No, that logic makes perfect sense. Firms can charge whatever they want. If you cost more to fly, then firms can charge you for that.

It costs a lot more to do a lot of things for disabled persons. To that end the US government passed the Americans with Disabilities Act, which dictated that most of those things should be done for the person at no additional cost to the person. Like most mandates, it acts a bit like a tax, in this case paid by the population at large, to benefit the disabled population. In the case of an airline, it's generally not a large expense - capex on a few airport wheelchairs and opex in the form of someone to assist in the physical transfer of the disabled person.

The ADA is generally regarded as a generally good thing, especially insofar as it influences the design of new construction to be more accessible to begin with, but it's not without flaws (e.g. turning disabled persons into liability factories - http://www.mercurynews.com/2016/04/10/serial-ada-lawsuit-fil... ).

It's far less clear that we should levy a similar mandate on every business ever in order to mitigate every single difference in life circumstances. In the case here, it should be quite reasonable for airlines to charge people for a portion of their carriage by the kilogram, like they already charge people for extra leg room - it'd make tons of sense, and the only reason that they don't do it everywhere already is because a standardized price makes the transaction easier to accomplish.