I'm not sure there's a difference in photocopying an entire book and doing the same to a DRM book. Do you know specifically that there is a difference?
EDIT: Specifically, it is against the law to break DRM (unless for few cases outlined by library of Congress), and the law as written makes no exceptions for fair use. The courts have also held in positive that it was Congress intend to make breaking DRM a crime even if there is a fair use case.
Lets not lose sight of the fact that DRM does present a different situation by pulling additional laws into play, in our blind defense of DRM.