Plagiarism is a glaring mistake, but people always make mistakes.
I'm surprised at such fairness but I think the most important thing is that a person recognizes his mistake and asks people to forgive him.
The mistake should have been recognized when the user previewed their post (or looked at it afterward). If you're worried about "haste" and are posting too quickly/too much to even give it a look-over then you're also spamming. The punishment is warranted.
Surely, we don't post on the forum without looking over what we type, right?
1200 messages, eternal ban because of one mistake it's like something too much
Plenty of users with higher post counts have been banned.