Itachi being passive in this case could also arise from the fact their mission was more likely to delay the Konoha gang than to really defeat them. If they had aimed for total annihilation, they wouldn't have used some other people's bodies that obviously limited their powers. I don't think even Kisame was arrogant enough to think it would be that easy. So, that being said, the more time Itachi spent just staring down his opponents or exchanging words, the more totally free time bought it meant for sucking out Gaara's demon.