Let us accept this reality- NE are people who are not part of mainstream in any way and love to follow Korean culture more than mainstream Indian ones.
Pray tell me, what is it to be a part of the mainstream? Who is part of the mainstream? Or is your idea of "mainstream" limited to the Northern plains? What history are you tracking down to delimit what "'mainstream" is?
Secondly, if you take a look at the map, especially with cultural significations, you might happen to notice that the idea of "India" does not have its basis in a reductive homogeneity. We are not a nation obsessed with tracing common histories, for our histories have been as varied as our cultures. Your idea of history, which you perhaps remember from school, is just the history of North India, with some sympathetic bits of the peninsula thrown in. If your idea of a nation is qualified by the sharing of a single mainstream culture which defines the collective that the nation represents, then I'm afraid your premises will not find much of a nation in us.
The idea of India is plural at its very roots, in its very inception. The idea of India is not a construction along the lines of sectarian identities. The idea of India is a possibility, a possibility of a pluralism that endures.
To posit otherwise is not a flaw in content, but in principle.
Also, where did Korean come from? Do you become American if you eat from McDonalds, drink Coke and shop from Walmart? Do you become Persian if you have a taste for Iranian cinema? Do you become European if you learn French?
A nation is just not a federation to extract money, it is based on common history and culture and on these counts, NE ones themselves feel different.
However groups which are Hindu are part of mainstream like Assamese and some Manipuris. Nagas and Mizos are as much closer to me as a Korean.
To protest against the State is not to denounce the idea that the nation represents. And the people from the North East have some very good reasons to be involved in resistance. I am not too sure how you'd feel if your family was lined up on the streets and shot dead, because someone else from your community was involved in Anti-State activity. Ours's is not an autocracy, my friend. This is not Nazi Germany, nor is it a ground for Stalin's purges, nor is it America's McCarthyism. A voice of dissent is not the voice that must belong to an 'Other', so conceived. If that's what you believe, then the idea of democracy has not dawned upon you, yet.