They do not have to if they don't wish to. This ethno-fascism is outright deplorable. They are a group with different culture, traditions and practices and as a democratic nation it is your responsibility as majority to not enforce your beliefs upon them.
Do you know what binds India's northeast together, a place your kind often deride as being less developed yet try to crawl in at the first opportunity. It is the government's policy of recognizing the uniqueness of each tribal groups and allowing them the opportunity to self govern themselves.
As a result, you have the autonomous councils for the Bodos, Dimasas and the Karbis among others. And that is precisely what differentiates Bangladesh from India.