Your definition of service is incorrect:
(From
Wikipedia)
In economics, a service is an intangible commodity. More specifically, services are an intangible equivalent of economic goods.
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If a person needs an additional seat in order for their "service flight" to destination X, it is not an additional service. If you take the time to understand the definition of "service", this is apparent.
There are motorized wheelchairs that weigh well over 300 lbs for people with physical disabilities---and no, these are not bariatric chairs. If the airliner destroys the chair or something like a ventilator, they have to, by FEDERAL law, pay the individual who uses the equipment the purchase cost of the equipment--not the current value of the equipment. You think it is fair to the healthy customer if the baggage people don't stow away equipment properly and then they have to help pay for a $20,000 chair and a $30,000 ventilator because the airliner was not careful enough? Should the airliner defy the ADA and not let people with physical disabilities on the planes because of this?