It's an interesting issue,so thanks to the OP for raising it.

I guess it is up to the airline to make the decision -and without defending the airline -a blanket policy at least reduces the chance of prebooking an UD seat and then being told "you can't sit there" on the way out, but on the return "no it's fine". At least, in theory, everybody knows where they stand with a policy. It would be unfair to expect check in staff to determine whether a wheelchair passenger is suitable for UD seats, and it could end up causing seat reshuffling with an hour or two prior to departure, which is never good!

I guess the lower deck it has to be -even if not ideal.

The mobility assistance issues are real -but I guess again that as others have said, you have to work around the airport and their systems, instead of the other way around.