The issue here is more "why would we ban someone who needs help." And as I think I pointed out, the fact that their issues can affect other members of the forum is the answer.
Your question, and only a part of the answer.
Not everyone can be helped, nor is everyone in a current place to be helped adequately.
If you want to use Sea within your post, then you need to use BPD within your post as being the primary cause.
Many do not understand the severity that BPD can have. I know there are people here with it who function just fine. I would take a stab that many here with that diagnosis more than likely don't actually have it, and it is misdiagnosed. Sea is not one of those cases, and has BPD.
BPD can literally be uncontrollable. Sea has been on staffs watch list for quite a long time, and Sea has done many right things even in his state by what has been asked of him, being to keep his posts in his diary only because he is not fit to post on the forum elsewhere at present.
Now removing Sea from the equation, reenter BPD. Anyone who displays severity of BPD to an uncontrolled state in the past, has been banned. They have also been randomly allowed back a year or so later, giving time a chance for them to have sought the local, professional help they need, of which this forum cannot give them. This forum is more than likely causing more harm to someone with BPD than doing good, when at that severity.
So... to completely answer the question of
"why would we ban someone who needs help?" The answer is because not everyone can be helped via this means. This very means can be more damaging and destructive, than helpful, to certain persons.
PTSD Forum is not here to help everyone, that is the simple truth. It is here purely to help a majority of people who have reached a stage of wanting to help themselves first and foremost. Nobody can help a person with PTSD, unless that person has decided and committed they truly want to help themselves.