Its a tricky line for sure. I think that punishment in a BDSM relationship has a place. When you consider that the roles take on at times a parental manner, and after all if a parent punishes its not abuse, then its a different ball game. Intent, consideration of boundaries however extreme is what makes it either abuse or not. If I punish a sub and its all part of the roles we have, if somehow they need or accept it as due and I take care to ensure that no harm is done I would not call it abuse.
If someone "punishes" under the guise of control, put downs and does not ensure to take good care of my mental health or physical health I call that abuse no matter why the punishment occurs.
I feel that a true punishment, not a play one where there is no real offence and the punishment it part of the game, but a true you did wrong and here is your atonement and penance can be a very interesting thing for both parties. I think it can even be healthy, guilt freeing and allow for acknowledgement of feelings or new direction in a relationship. To be penitent and then free from having to worry about it any more because the price was paid can be healthy. Look at religion and confession ... after all its punishment for deeds and sins as approved by the church.. law and order...where its court ordained and I am sure the punished do not want it and may call it abuse too. Justice and rules are in BDSM just as anywhere else, they are just negotiated on a personal level.
With the complexity that a BDSM life adds to any interactions I still believe the acid test lies in the feelings that the sub or Dom has with the interaction. If they feel abused and worthless its abuse. If they have to ask if it was, it was. If the Dom knows lines were crossed and does not care, he or she is an abuser.