Tim Wu explains the “fair use” clause in American copyright (found via), discussing Shepard Fairey’s Obama poster (it just won’t go away). In a nutshell: “Fair use […] is all about justification, and this is a key to understanding it. Fair use allows use of a work that would ordinarily constitute infringement, if that use is justified (or excused, if you like) with some compelling reason.” (emphasis in the original)

And about Fairey: “Shepard Fairey’s case, setting aside his recent troubles for a moment, is one of these new areas. To ‘Warholize’ someone else’s photo (if that’s the right verb) doesn’t fall within an existing category of fair use. So the question is whether it should. […] Is there some public interest in infringement here that justifies what Fairey has done? In the big picture, answering that question means weighing, on the one hand, the broader freedom to produce art like this—the freedom to be Fairey—against the sense that too much of the original was taken without permission and that the original photographer or his employer, the Associated Press, deserves to be compensated.”