Some time ago, I wrote about the ever-increasing extension of the term "plagiarism" to cover more and more behaviour, at least in the academic world. I just came across another example of this. Victoria University advises its English Lit students what to do if they discover that somebody else has already written about an idea they've had:
Your approach will never be precisely the same as the writer who has so irritatingly come up with the same idea, so you can stress the differences and acknowledge the other writer appropriately in your citations. Failure to do so constitutes a form of plagiarism, even if you honestly arrived at your conclusions independently.
No longer is plagiarism the deliberate attempt to pass off another person's work as your own. Now, according to macademia ("May Contain Nuts"), plagiarism can be the failure to cite works which had no influence on you because you didn't learn about them until after you developed your ideas.
In my opinion, the academic rules against plagiarism are less about preventing fraud and more about creating an insular in-group with rules of behaviour that outsiders simply don't understand. To outsiders, fraud has to be fraudulent to be fraud, and failing to say "by the way, this other guy had a similar idea, but it's different from mine" might be impolite and even incautious (in case others discover that work and wrongly imagine you copied it), but it isn't fraud in and of itself. But to macademics, such a failure to mention an irrelevancy is itself fraud.
If your mind boggles, its supposed to. It is no surprise that it is liberal arts departments that are prone to "scope-creep" of plagiarism and the application of other arbitrary rules. Like the rituals and secret handshakes of Freemasons, such practices are there to separate those who have passed the initiation from those who are merely smart and educated.