I think there are several levels of severity in plagiarism and not all are equal and in some cases a permanent ban forever can be a little harsh (especially if it was just one silly mistake)
How do we determine it's a single case of a copy and paste? Let's say user X has 5000 posts and he is caught and reported for just 1. How do we know that there are no other plagiarized posts? Who will audit these many messages and prove that it's "just one silly mistake"? Note that different plag. checkers give different results. What you get with plagium and seotools may differ from a quick custom google search. Then there are the spinning tools which are very hard to catch, let alone many other ways to fool detection.
I can't believe that there are five people who voted "Pay some sort of fine". That is some bullshit!