Majority of users know why they are banned. As they are probably guilty of pliagarism, ban evasion or some other violation.
I'm not so sure about that. For one thing, most of these copy/pasters don't read the rules before they embark upon their career of stealing other people's words, and for another they probably don't see plagiarism as doing anything wrong.
You have to realize that a lot of these bounty spammers who resort to plagiarism likely don't have much schooling (that's my guess anyway) or experience in situations where they'd find out that you can't just copy someone else's writing and pass it off as your own. Some of them are probably very young, too, and they might not have learned this fact.
And then there are those with no moral compass who will start ban appeal threads even though they know what their offense was and what the consequences are. They either want to beg for mercy with an excuse or hard luck story, or else they just want to waste everyone's time for the sheer pleasure it gives them.
Even if every ban that was issued had a reason attached to it in big, bold letters we'd still have these idiots posting in Meta wondering why they got banned. This is why there really should be a child board that deals only with ban appeal threads.
Edit:
Considering how many accounts were banned because of copy and pasting, it may be worth to add warning in the field of sending message.
Despite what I just wrote about such a statement likely being useless, I still think it would be worth adding to the ban message. It couldn't hurt, and it would probably eliminate the need for people having to find examples of the person's plagiarism when they start a ban appeal thread and ask what the ban was for--even though I'm pretty sure they'll still deny they copied content and will still act just as baffled that they got banned as they do now.