Judgment of Plagiarism

I'm at the beginning of Richard Posner's "The Little Book of Plagiarism" and I ran into this passage (locations 138-145 of the book¹, emphasis original):

A judgment of plagiarism requires that the copying […] induce reliance by [the intended readers]. By this I mean that the reader does something because he thinks the plagiarizing work original that he would not have done had he known the truth. […] If [the reader is] a teacher he gives a bad student a good grade, to the prejudice of other students in the class (if the students are graded on a curve), thinking the student's paper original.

I'm citing this passage because I believe many of us, in our early student days, had this experience: you wrote something that the rest of the class copied, but you got a mediocre grade, if not the worst.

Many years ago I helped someone write her Fortran 77 homework (which was taught in her department of some engineering school, using g77), and many of her classmates copied her homework, some by copying the built executable, some by copying the printout of the source code, which carried errors. Surprisingly all those copiers got better grades than she did (some 90s, some 80s, out of 100, and she only got 70 something).

It was a very comical experience, not least because I was the ghostwriter. I tend to think I never became proficient in Fortran (or decided not to be) because of that episode.