and the benefit to IB of doing this would be??
Remember it is just IB that is affected by this, you have to look at this overall in terms of IAG/LATAM/OW, LATAM can have free regin on routes from their hubs to MAD and other European destinations, while BA can launch routes from LHR to LATAM hubs using IB slots at those airports (Latin America is poorlly served from the UK)
Or to prevent IB from cutting service to Latin America, maybe LATAM can agree with IAG that flights to and from LATAM hubs-LHR/LGW/MAD/BCN are to be served (for OW) BA/IB, in return LATAM gets to serve the rest of Europe from their hubs