On codeshares it is indeed the Marketing airline (IB) that has to approve the TPs and Avios. So for example BA have to approve any TPs on their codeshares within Spain even though BA doesn't operate Spanish domestic flights. Now you don't personally approach IB, you submit claims to BAEC, but under the bonnet, it's an IB decision even though they don't operate LHR-PHX. But you can see the difficulty here. Let us say you had a BA codeshare on some Spanish domestic flight run by Iberia, and you had an irrop, so you were diverted via (say) TAP instead. BA would have zero line of sight to the original problem or the solution, so to approve the missing TPs they would have to make complex enquiries and/or look at boarding passes/e-tickets/PNRs and analyze them.