That's why they fail. Pain should have thrown a few parties in the hideout, let his emloyees know each other better and resolve old quarrels...then they would be a happy family and no one would defeat them.
Why on earth would Zetsu help Itachi?
This is Akatsuki, not your local football team.
Zetsu answers to Madara, not Itachi. He is probably shocked about the outcome, but why would he care? He's a plant for God's sake. Interfering with this fight would go against Madara's wishes, who I presume wanted Sasuke to win.
Im over it already,i thought about it,the only people in Akatsuki that actually give a rats ass about its goals is pein and madara,hell i would'nt interfere if i was Zets! guess he just wants some grub!
Everyone in Akatsuki are self interested actors, they do things for themselves. Perhaps it is in Zetsu's interest to be an observer. Even more so, Itachi is finally down, along with some of the other strongest fighters, for the count and the villages are now aware of their existence. It would probably be in Zetsu's to pack his bags.