There is no excuse for discriminating against passengers with disabilities. I believe that airlines and other transport services should not only do everything reasonably within their power to make their services available to everybody, but that this should be done without additional cost to the passenger who needs the support.
However, there is an issue of "reasonableness". A certain airline has been criticised in some quarters because of this case. In short, the airline has expressed reluctance to carry wheelchairs weighing over 60kg for passengers, unless they can be dismantled into manageable sections. On the back of this, "all airlines" are being called upon to change their policy and make it easier for disabled people to fly.
Let's be clear about what is being asked of the airline. A chair that weighs 120-140kg weighs as much as seven heavy cases, and about four times the maximum permitted weight for passenger cases (and as much as two normal-sized people!!). It is also not designed to be manhandled - when it arrived from the factory or store, it would probably have been driven off a lorry, not lifted off. At the owner's house, it would stay on ground level. I have tried to carry one - with three other loaders - and it was almost impossible - as much as anything else, there are no handholds to facilitate it. They are built to move around using their own propulsion system. The passenger's chair is more expensive on its own than the luggage of the average family.
Travel is inconvenient for everybody: nobody really travels "their way" unless they are a millionaire. Airlines have arrangements in place to carry passengers with reduced mobility - including those who are paraplegic. Given this, it seems sententious to me to argue that the airline is discriminating against disability because it is unable to easily transport something that was never designed to be transported in the first place. It is as much the responsibility of the passenger to work with the airline and find out how they can be accommodated. Passengers with specific requirements who realise that they are making unusual demands and are polite about it are generally treated with respect, sympathy and consideration, and generally staff (and other passengers!) will do what they can to help out. People who are perceived to be making unreasonable demands as "my rights!" may get what they want in the short term, but end up alienating themselves from others and hardening attitudes against themselves.