To BA, I imagine, as there must be an actual or notional kickback to the airline, and to the customer who is willing to pay extra for more miles.

I usually check various permutations, and the BA/Avis link usually works out more expensive than making bookings separately. Airline customers seem to be captive customers to the rental companies, a bit like shopping for batteries on Saturday afternoons when Dixons (er, Currys Online) are still open.

Even without promotions, I get 1,000 VS miles per Avis rental and a better price using another discount code. Best (for me) in the US seems to be using National promotional rates earning AA miles. As always, YMMV.