It does seem to be the commercial policy - during the strike threats they were still refusing to rebook on codeshares, just BA operated flights on different days/BACON ones.
I am not surprised AY helped you. They rock. They did the same for me during the August security chaos - moved me on an I class to the delayed flight which was going next, no fuss, no issues ^ FWIW on that trip my flight posted as the original one not the new one with correct points/miles. Weird.
I suspect you're in for a fight with BA