Yeah you have a point there, but don't you think that is not the case when it comes to Pakistan, especialy for those who migrated to Pakistan at the time of partition? Lets not forget, it wasn't a typical migration (like Afghans did during Afghan war and then settled in Pakistan and mixed so well with PAKISTANI pukhtoon population that now it's really hard to differentiatee them). Since this was NOT typical mass migration hence typical migration rules does not apply. They KNEW when they were migrating that they are leaving everything behind; that included culture and what other people call "ORIGIN" and "ROOTS". I don't think those who left their father's and forefathers graves in India for Pakistan in a whim would liked to be linked with INDIA instead of Pakistan. Nor would they appreciate if some1 start associating them with India instead of Pakistan (For them it's like Pakistani Pakhtoons start associating themselves with AFGHANS, instead of Pakhtoon areas which is part of Pakistan. getting my drift here)
I think we need to distinguish between national Identity & cultural association. From the perspective of national identity you are absolutely right, it is invalid to associate them with Indian identity based on the choice they or their forefathers made at the point of creation of Pakistan, but cultural identity is a different thing.
A pakhtoon living in Mardan is as much an heir to the Pakhtoon culture as a Pakhtoon from Kandahar.
& I believe the same applies to people who migrated from different parts of India....