Just cause you picked duty roulette doesn't mean the other people did. The duty finder matches people reguardless of if they picked DR or they just queued for specific dungeons. There are not many exp type mobs in the game to spirit bond.

Its more of a problem with spirit bonding in general then people doing it in your duty finder run. Honestly the system should be reworked so that you don't have to use crappy gear and kill mobs that give exp with it on to get materia.