I don't think Plagiarism is something that needs to be pointed out as being something you can't do. First of all, this is something that people should know they shouldn't do, for a number of reasons, and I believe most people who do this know it is wrong, but do it anyway.

Also, I don't think it is as big of a problem in terms in the number of people getting into trouble doing this. Sure there are very many instances of this happening, however I suspect this is done by a fairly small number of people with a very large number of sockpuppet accounts, and I suspect this is often done via some kind of automation/AI.