I think the reason (which is started in the PD) is that something is no longer just is when it's no longer mutually beneficial and no longer adheres to the "basic grasp of justice" which, as I see it, is to neither harm nor be harmed.
Ok this is going to help because I think we need to focus on this issue, which I was starting on when I called 'harm" ambiguous:
Who gets to decide whether some is being "harmed" or not? I don't think there can be much of any absolute standard on that, and it's difficult to decide where the limit might be, given the rest of Epicurean philosophy.
Because I think we all end up at the same position if we agree that "harm" is very subjective and relative. If we think "harm" can be defined objectively, then we're looking at an absolute standard of justice which I don't think Epicurus would allow.
In the end, the only thing that is desirable in itself is pleasure and the only thing undesirable in itself is pain, so just at there is no objective "good" but pleasure, there is going to be no objective "bad" but pain.
Agree or disagree?