I think you're wrong. Psychopaths, at least, don't learn from consequences because we are physically incapable of it - psychopathy is a change in your mental wiring that results, amongst other things, in a lack of the ability to feel fear and learn from (and internalize) consequences. This is why psychopathic criminals are the ones that are almost impossible to reintegrate into society. While people with HPD aren't necessarily the same, I think that the fact that they seem to never really learn from consequences (except for very specific ones) isn't just because society lets them get away with it but rather because they can't.
I disagree on it being used as an excuse, while I agree on your content.
Yes, I am aware that the "wiring" is different. As I have written elsewhere on this forum, especially, the "wiring" in terms of the outer world feedback loop is impaired (empathy), resulting in the lack of ability to integrate this outside feedback and consequence with their internal world. This is why the understanding of the consequences are lacking. (In my previous post I wanted to keep it simple and avoid this kind of content). Again this "wiring" is also a psychological and behavioral construct to understand the thought process - there is a lot of debate on how much of it is reflected by "actual" neurological evidence.
However, not understanding consequences doesn't mean Cluster B's don't recognize their behavior as inappropriate. Many of them, especially the high-functioning ones are sufficiently intelligent to know that their behavior is inappropriate. The fact that all Cluster B's use "masks of sanity" is because they know how normals behave and they SHOULD behave. However, they are able to use their "primitive" defense mechanisms like denial, disassociation and twisted rationality to justify to themselves their own inappropriate behavior, and then use lies and distortions to convince others. The fact that this is a habit from childhood makes it easy to use and difficult to give up. The fact that normals believe them only enable these.
Normals control themselves by facing consequences, pain and postponing short-term gratification, in favor of the long-term. Cluster B's don't postpone short term gratification. Yes, some Cluster B's, especially low-functioning psychopaths may not improve with therapy, because their "wiring" is impaired beyond repair. However, sufficiently high-functioning Cluster B's, when faced with the inability to cope with their life and wanting to improve, do seek therapy and improve.
I have written elsewhere on this forum, that I have myself faced insidious childhood abuse, and could have grown up to be a psychopath. However, in college I recognized my behavior deficits and actively worked on them for the last 10 years to improve them. If I merely recognized my deficits but didn't work on them, excusing my behavior saying that my wiring is faulty and can't do anything about it, I would have been where I was - I would have rationalized my behavior as "survival of the fittest" or some such twisted logic and carried on. Rationality is the last refuge of scoundrels - and when twisted, offers them sanctuary to escape. But I CHOSE not to do it and instead work on myself - facing all my inner demons and fears with the same brutal honesty that I had previously applied onto others as an psychopathic tendency devoid of empathy, while on the other hand literally re-wiring myself with the outside world and its emotions.
HPDs, especially high-functioning ones, can improve through therapy, when they realise that they NEED and WANT to improve. Unlike psychopaths, HPDs are basically "fearful children" and the "reward and punishment" mechanism, which they understand very well and which they effectively manipulate, if used effectively against them by people who matter to them, could help them atleast get into therapy. For this they have to be held continuously accountable until there is a NEED and a WANT in them to seek therapy for themselves.