Following your same logic, what makes you think that the Constituent Assembly will work if referendum on this issue can not? Wouldn't the same leaders be elected to the Constituent Assembly as well? How will this be different than the parliament that can change constitution as it is doing now? Let me tell you my answer - Maoists will force the people to vote under their gun point and if they believe they can't then there wont be constituent assembly election, resulting in more chaos. By that time the Maoists will already have dismantled all the organizations that could fight against them - Army, SPA, Parliament, Hindus, etc. Note that they have already said they dont want the local bodies to be reinstated, and they want parliament to be dissolved as soon as possible, makes any sense?