I queried this yesterday on Twitter and received this reply. I tried many dummy bookings to the USA and I cant see anything in the T and Cs for BA, BAEC or on the Speedbird Club site but I'm sure there was a clause stating this before
I do hope it's a glitch!!!
Hi there PETER01, We're not really able to help with quotes but as far as I remember, you're now charged an Avios amount for domestic add-ons. It's been like that for a while though so not sure how long it's been since you last booked?