Again, I'm sorry that you are so upset about your experience.
Delta, American and USAir can ask you to request cabin stowage ahead of time, but per the Air Carrier Access Act, they cannot require such advance notice in order to provide legally required accommodation. The only thing they can require is that the passenger requesting cabin stowage pre-board.
In my case, there was no other passenger on my flight with his/her own wheelchair, so I wasn't depriving any other wheelchair user of cabin stowage, requested in advance or not.
I also wasn't flying Delta, American or USAir, but that's moot.