Why Is Plagiarism Bad?

A question has been going through my head, inspired by two unrelated happenings. The first is the deplorable aetosaur scandal that has been making blogging news. The second is that Jack and I are going to see a performance of Richard III this evening. What do Spencer Lucas and William Shakespeare have in common? The answer is that both of them have been accused of plagiarism.

Whether William Shakespeare actually plagiarised his works in detail remains a debatable question (see here, here and here, for instance), but one could certainly argue that Shakespeare did not invent much in the way of original plot lines. Fans of Shakespeare would nevertheless reply that even if that were true, the skill with which Shakespeare built on and improved his original sources surely makes up for his initial "borrowing".

The point I am considering is that our definition of what counts as "plagiarism" seems much broader now than it once was. I'm not referring specifically to the Lucas case in any way here - if the accusations are true, then that probably counts as unethical whatever period you look at it from. Besides, the standards of science are a little different from those of literature. It's just that the question of "what is plagiarism?" has been on my mind as a result of the case.

Herman Melville's Moby-Dick, or The Whale and Edgar Allen Poe's The Narrative of Arthur Gordon Pym of Nantucket were two books that retained their popularity and influence despite large sections of them being derived from other works. For instance, Poe lifted several passages out of Benjamin Morrell's A Narrative of Four Voyages, without even correcting typos. How were Poe and Melville able to get away with it?

One possible explanation that occurred to me was that maybe such extracts were more tolerated in the past was because plagiarism was actually harder to get away with in the past. The pool of authors and readers would have been much smaller in the 1800s than it is now, and as such, the chance that a reader would be familiar with a given source work would have been much higher. Did Poe feel able to draw from Morrell without the need to indicate he was doing so because he felt that most of his readers would have read Morrell and recognised his borrowed passages as such themselves? Whereas in the modern day and age, so many thousands if not millions of pieces of text are floating around out there that it would potentially be much easier to find some obscure source and pass off the work therein as your own. Does increased potential to get away with plagiarism mean that penalties have become higher to try and deter people from trying their luck? What do people out there think?