Same reason why dhaka/any other part of Bangladesh is part of Bangladesh... Answer that question and our reasons are pretty much the same...
No, no. Case is completely different in case of NE. The annexation of entire Assam by the British Bengal Presidency was completed only in the 1850s. NE was independent before. So, the colonial Union govt in Delhi should vacate NE as soon as possible.
This will also help BD, because India will have no reason to ask for transit from us. All will leave in peace.