Judge Richard Posner wrote, earlier this week, a concise and short essay for Newsday about plagiarism:
...Plagiarism can be a form of fraud, but it is no accident that, unlike real theft, it is not a crime. If a thief steals your car, you are out the market value of the car; but if a writer copies material from a book you wrote, you don't have to replace the book. At worst, the undetected plagiarist obtains a reputation that he does not deserve (that is the element of fraud in plagiarism)....
...Plagiarism of work in the public domain is more common than otherwise. Consider a few examples: "West Side Story" is a thinly veiled copy (with music added) of "Romeo and Juliet," which in turn plagiarized Arthur Brooke's "The Tragicall Historye of Romeo and Juliet," published in 1562, which in turn copied from several earlier Romeo and Juliets, all of which were copies of Ovid's story of Pyramus and Thisbe....
...Judges, who try to conceal rather than to flaunt their originality, far from crediting their predecessors with original thinking like to pretend that there is no original thinking in law, that judges are just a transmission belt for rules and principles laid down by the framers of statutes or the Constitution....
The whole essay's worth reading, so go check it out.