"What does BA gain by this practice?
Money, money , money... that's all they think about these days
I agree. It seems to me that if you book advance purchase discounted fares they will crush you in some way, even Business Class. For example, I have seen airlines charge for seat selection and also for additional baggage. It usually works better for higher frequent flyer classes but not always. Their algorithms are varied and inconsistent, especially when it comes to code-share flight bookings.
I've concluded the only thing is to go into the booking process far enough to see all the inclusions (more particularly if you can see the exclusions) and be able to escape before you pay if you don't like what you get. However I have been trapped and had to cough up extra for seat selection after paying for the flights and it irks me but what can you do?
They are trying to make extra revenue at any chance they get, so all the old expected rules are out the window. Buyer beware. Change airlines if you can verify a better deal. And try a good travel agent - sometimes they can come up with better than a direct deal!