Originally Posted by superninja
I won't go into whether he is really Madara, but he did use the name "Tobi", so Zetsu calls him that way. It could be that Zetsu likes that nickname. Zetsu is the closest associate of Madara/Tobi so either he calls him Tobi because Zetsu knows the guy is not really Madara, or he calls him Tobi because he likes to call him that way or is used to calling him that way.
I agree well said superninja.