To answer the question: because they provide a valuable network to One World and get valuable business from One World. The new MAD airport is set to become/is already, a rival to London and Paris for connections.

By the way, I took an award flight IAD MAD IST recently and liked it very much in IB J class. And, the IB lounge in MAD is really something, sleeping quarters and a Spanish wine bar with a wine steward. Yummm.