Good question. Nagato didn't know about the Moon's eye plan, if he had known he would'nt go for it. His plan was to enforce fear through the threat of terror using the Juubi as a catalyst.How was obito/madara plansupposed to work!!?? Nagato believed that the guy in the mask was Madara all the time right!!!?? and if Nagato knew about the plan, was he really gonna sacrifice himself!!?