The airline cannot magically produce a wheelchair or a trained member of staff to handle the passenger or the wheelchair, so what must they do if the service provider doesn't adhere to the contract? The passenger will suffer, but it most certainly isn't the fault of the airline.
You Europeans need to fly in a place where service means just that. From the check-in counter to my seat to being put into the terminal at the other end, here, I'm the airline's responsibility. Still if you only pay $10 to go 500km, what can you expect...