I totally agree (assuming it's an organisation not a person that awarded it to you), however, what if you left on good terms but perhaps had a difference of opinion on the organisation direction? Is that any reson to give up on your awarded ranks?
That's a great hypothetical. If the organization is happy for you to retain the rank then that seems fair. I don't think this is the case, however.
My opinion is not legal, nor technical in any way. It is simply that personally, I would not want to claim a rank from an organization that I do not respect and has specifically asked me not to. Similarly, I would not respect somebody that did such a thing, and I would not want to join their dojo.