I seem to recall from my days of Russian coursework 20 years ago that only US-chartered airlines are permitted to fly nonstop between the US and Russia (then the USSR).

If the same is true of other countries (e.g., only a British-flag carrier may fly nonstop between the UK and Russia and a German-flag carrier the same between Germany and Russia) then an AA codeshare from those countries might run afoul of such a rule. Clearly, just my rampant speculation.