I think there is a grain of truth in what aa are saying. You can book into economy if there's no availability, and because of the way the XUPP fares work over at aa, I wouldn't be surprised if the system did the downgrade automatically on D fares. They probably get treated in the same way as discount first in the US.

Bottom line is they didn't actually sell you a biz seat, and you got what you expected. You just happen to know what DEUNBA means...