It isn't always possible to figure out from the public data why an account is banned.
When a banned account's comment gets vouched for and rescued, we look at the account history and unban it if we see that we banned it unnecessarily (we try not to do that but it happens) or if the commenter has reformed. Alternatively, there may be a good reason for the account to stay banned, but if it posts good comments, HN users will vouch for and unkill those. This system has proven to work well, even better than we hoped when we introduced it.
When a banned account's comment gets vouched for and rescued, we look at the account history and unban it if we see that we banned it unnecessarily (we try not to do that but it happens) or if the commenter has reformed. Alternatively, there may be a good reason for the account to stay banned, but if it posts good comments, HN users will vouch for and unkill those. This system has proven to work well, even better than we hoped when we introduced it.