I would imagine that if a lone firefighter shows up at a fire, that person had better be able to do everything. Is that how fire operations work? The ones I've seen involve a lot of people, and some are operating pumps and holding nozzles, doing many things for which the strength to carry a wheelchair-bound victim is not a requirement. It sure would be a good thing if at least one of them is a strong man who can carry someone who cannot be dragged. But if a criterion excludes a huge majority of the population, how likely is it that the remaining one percent will want to become firefighters, and how much are you willing to pay to convince them to?
I'm under the impression that fire fighters, at least professional ones, do require some training, which includes at least a modicum of strength as well as rescue techniques. I really don't know, but I'd be interested to see some reliable statistic reporting how often the inability to carry victims has been a problem, and how often such a problem can be blamed on diversity or could have been averted by the presence of a person of normal human strength.