You specifically mention AA/BA. AA/BA have a specific relationship which starts with BA's generous policy that is more than the law/rule requires and more than any other airline offers. In essence, BA looks at the maximum possible allowance that any 1 segment offers you on your entire booking (both directions, not just that journey), and offers that allowance for your entire booking.
So if you were booked ORD-LHR-DUS and then DUS-LCY, and then LHR-ORD (completely made-up itin) where every single segment is in discounted Y and you have no status - EXCEPT DUS-LCY is in Club Europe (C). In that case, your allowance for the entire journey would be 2X32 kg and not 1X23, even on the outbound.
Then, BA and AA (and IB and QF) have a special relationship, more than the regular OW codeshares) that specifically offers the higher allowance across all segments for a one-way flight between the airlines. So when flying JFK-LHR-MAN for example, AA connecting to BA, 1st segment in Y and 2nd segment in C, you get the C allowance, even though DOT rules don't require it, IATA rules don't require it, and OW rules don't require it.
Certainly nice for the customer, but AA doesn't do that with other partner airlines, and other airlines (UA, DL, etc.) certainly aren't bound by that (much as we'd all might wish those to be the rules)