I'm not saying there is any mechanical difference with the cards currently available...
As you said your self, all cases of "the defender" require you to win/lose. The main gray area being the cards using the term "defending player".
The primary suspects being war of attrition, burned men(not so gray), reek, doran martell & pickpocket, which i'll suppose you'll agree all conveniently comply to my definition of "defending player".
All that combined with Nate's description of "Defending player"...
Whether or not this was an intentional difference by the designers? I'm thinking probably not.
If there are any "defending player" cards which apply to supporters, please bring them up.