Well, I think that saying, in effect, "everyone does it" misses the point. Yes, plagiarism is commonplace. It can even be flattering ("imitation is the sincerest form of flattery", that sort of thing). However, that avoids the question of whether plagiarism is to be tolerated, and if not, what is to be done about it.
My guess would be that most of the code that someone steals is considered "free" as in speech, available to everyone, much as Perl itself is. In that respect, you do have a valid point - if it's freely given away, why complain when someone takes it?
In my opinion, the answer is that the taking is done without proper "thanks", so to speak. It's one thing to take examples out of a book or from Perl Monks, without attribution, to use in programs you write. You probably couldn't find the proper attribution if you tried; and who tries? It's another matter entirely to take a whole program published elsewhere, copy it verbatim or with only cosmetic changes, and present it as your own. It's dishonest. I suppose I could take the Perl language source code, remove Larry's name, and replace it with my own. As far as I know that would be legal, maybe even within the license - but would it be right?
XP on Perl Monks has whatever a person gives it. To you, since it can't buy coffee or impress the ladies, it means nothing. Me, I have the opposite problem - the ladies are only after me because of my XP. It's a curse, I tell you... but I digress. To some people, XP here is really important, worth cheating for.
I don't see that anyone wants to set up the Node Police. I think that some people were thinking of offering some 'encouragement' to everyone to keep it honest, by providing for sanctions if they did plagiarize.
Still, maybe the best solution is to point out the instances where posted code or text appears elsewhere, indicating where that material may be found. Let the community decide if it's plagiarism, and if so, whether to downvote or otherwise comment. Perhaps peer pressure alone will be enough to discourage dishonest behavior.