Please see my answer below. I have incorrectly applied my Wikipedia experience to Stack Exchange and based my assumptions on biased data, and I am deeply sorry for the controversy I have generated.
A follow-up to this question.
I find the attitude towards post-banned users to be very harsh, giving the message of "you're on your own" rather than "here are some suggestions on how to improve". Can we actually help users who are post-banned improve their posts instead of saying "tough luck")? Even if users are making low-quality posts, we really should be assuming good faith whenever reasonably possible, since these users generally aren't being malicious.
I hate seeing post-banned users crying "miséricorde" but not getting any mercy from the community. A lot of these users really want to and can improve. User behavior is not set in stone, and assuming that they can't improve is the wrong thing to do.
We should stop assuming that users know the rules or understand how to write better posts by the time they get post-banned. As Kyle Strand said in this answer:
We need to stop relying on our tools--the FAQ, the pop-up suggestions, and so on--to teach new users to become valuable contributors, and start teaching them ourselves.