Jeanricuser, you clearly forgot chapter 640. It's not Zetsu pulling this shit (specially considering his Zetsu part doesn't even cover the head, much less the brain to pull tricks like these), it's Obito himself (or rather, his subconscious) doing it.
First of all, http://www.mangapanda.com/naruto/604/8
confirms that its got a consciousness of its own. The zetsu share a hive mind and as this episode confirms its very much intelligent. You have also forgotten some simple facts. All the bodies of the ones trying to convince him to stay alive are part zetsu. Two the half zetsu rin seems to act completely unlike Rin which Obito notices. Three Obito's zetsu cells are not like the hashirama cells. These have a true consciousness all their own unlike the Hashirama cells in Danzo which were just implanted but rebelled when Danzo didn't have the necessary power to control those cells. Obito on the other hand has a zetsu which is an entity unto itself which can take on a number of different functions.