Is there actually time to forfeit at the end of the battle? I had this happen to me once recently and my last pet (Darkmoon Zeppelin) died. Then my bombing run came down and killed the opponent's pet. I don't remember having enough time to consider forfeiting before the game loaded me back out of the battle. I guess I'll see for the next time, because I do think that if it is clear who the winner should have been, then one should forfeit to allow the other to win the match. Unless they put in an achievement for "x" number of "draw" matches.
They could award a pet for it too...lol, now I'm just dreaming up ways to get myself more pets.