O'Rourke's argument, in a nutshell:
Behind the talk of originality lurks another preoccupation, less plainly voiced: a concern about the just distribution of labor. In plenty of instances of so-called plagiarism, what bothers us isn't so much a lack of originality as the fact that the plagiarizer has stolen someone else's work—the time it took to write the words or do the necessary research. The cribbed student essay—which Posner views as a particularly insidious form of plagiarism, committed by approximately one-third of high-school and college students—isn't an academic crime because a C student has tried to pass himself as a Matthew Arnold in the making. It's an academic crime because the student who buys his thesis from a paper mill has shirked the labor that his fellow students actually perform.
What really bothers us about plagiarism isn't the notion of influence itself, but the notion that a piece of writing has been effortless for the thief in question. Instead of worrying whether writers who borrow from other artists are fakers, perhaps we should be asking if they're slackers. It might make it easier to decide which kinds of influence to condone and which to condemn.
I think that O'Rourke raises a good point, but I still can't totally agree that someone who buys a term paper online has only committed a sin of laziness. McEwan's borrowing demonstrates that originality and attribution in fiction are troublesome things to quantify—How much borrowing is too much? How many words in phrase? How much of the structure of a plot?—but the world of nonfiction is somewhat different. Ideas and arguments can and should be traced and attributed. It is no sin to adopt and expand upon someone else's idea unless you pass it off as your own.