From a Tim O’Reilly interview (via All Things Digital):
People who don't pay you generally wouldn't have paid you anyway. We're delighted when people who can't afford our books don't pay us for them, if they go out and do something useful with that information.
I think having faith in that basic logic of the market is important. Besides, DRM interferes with the user experience. It makes it much harder to have people adopt your product.