Neither TK nor AI are in Oneworld; they are both in Star Alliance.
You are correct that once a bag has been checked through, no further fees should apply; it is not clear to me why the luggage was even an issue at an intermediate point. The applicable luggage allowance on a single ticket (on AI or TK) from USA/Canada to India is 2 suitcases per person (max 32kg each in J). This rule is listed for AI
here.
I have been harassed for luggage fees by 9W at DEL when switching from another airline to a 9W domestic flight (same ticket)---I had to stand my ground and explain the rules to them. The relevant words are "single ticket" and "luggage allowance from USA/Canada". If DXB-COK was a separate award, or if the luggage was collected and re-checked, that's a completely different matter.
I am not sure you have basis for a complaint, though, given that no money was actually charged. There is no point in complaining to AI about incompetence, there are too many such complaints.