I think perhaps its because the target market for ebooks is different than it was for music.
While there are bootleg sites for books, they haven't reached the masses like some of the illegal sites for sharing of mp3s.
As long as most people crack DRM so they can read, not to mass distribute then as others have said, the increase in sales negates the any lost sales.
This could change if eBook reading picks up on mass market devices (like the iPhone) and reaches the "why pay what you can get for free" market.
But for now, I think drm does what its intended for the most part (e.g. deters illegal sharing of books but isn't so unbendable that it prevents use).
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