Earn-only means they pay USD to AA (and AA gives them miles). AA likes that. Redeeming requires AA to pay them USD (and they give AA a seat to use for award travel); obviously AA doesn't like that as much. In any case it is AA that decides -- and you see the decision. No reason to expect it to ever change, or to think that any of these airlines will be joining OW in the next decade. It is just a marketing scheme for these airlines (like advertising).