Firstly, as it has already been mentioned in the present query that bank is demanding original copy of the NOC when there is no liability for you to provide the same to them.
Secondly, it is also true that the agreement of the loan would be interest eh favour of the buyer now not with you whcih makes it clear that you may have the liability to get the clearance from the previous bank, just to make it easy for the present buyer to have a easy loan from the bank.
Thirdly, this is not been the regular practice of the banks, but yes, it is also not wrong to say that they do that to be in the save position from any future incumberrance without knowing that it may not affect themin either way.