Originally Posted by shefgab

The difference between a HBO fare, and the corresponding non-HBO fare in the same class is minimal (around £20?). The changes affect all HBO tickets, even for the more expensive fare classes. You can be on a cheap ticket that includes a bag, where you can select a seat, or you could be on an expensive fare (which has a much better yield for BA) that is HBO. There is no correlation between the fare class and if it is an HBO or not. Expensive last minute HBO exist.

e.g. tomorrow morning at 0705, BA charge £497 for a HBO o/w ticket to FRA (Y class) - PAID seat for Gold/Silver/Bronze
But on Thursday at 1445 from LCY, BA will charge you £101 for a non-HBO ticket (O class) - free seat for Gold/Silver/Bronze

Where is the logic and link between fare, yield and HBO?

It seems BA have taken the arbitrary decision to change seat selection for HBO, where in fact they should have (well, if they need to do something) changed the seat choices for those on cheaper fares, where the deciding factor is fare class, not HBO/non-HBO. Checking in a bag has nothing to do with seats, and I guess not much to do with yield.