You don't have a physical book. You bought a license to legally own a duplicate of information. You didn't buy a physical product, or you would have the legal capacity to modify or recreate your product for non-commercial reasons. You don't have the right to redistribute it or duplicate it, so they are complaining about it.
It isn't that much their fault either, since it is inherent human nature and culture to share, and having grotesque IP laws that conflict with what your perceived normalcy is can potentially damage profits!
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Edit: I lied about the 30% fixed. I had JavaScript disabled and the sliders didn't show up.
They have this on their page:
"Zero DRM. There's no DRM on any of our books. None. Why? For one, we don't believe DRM actually stops people from sharing what they want to share."
It isn't that much their fault either, since it is inherent human nature and culture to share, and having grotesque IP laws that conflict with what your perceived normalcy is can potentially damage profits!