In this case it’s not so much DRM causing a problem but rather that the removal of it has had roughly zero impact on the piracy of ebooks from publisher Tor (book publisher, not popular
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Tor removed DRM from their ebooks a year ago and appear to have recorded pretty much no rise in the illegal distribution of their wears. Yes books are still being distributed for free by those that have no right to but that was happening even even DRM was still in place. Tor appear to have realised piracy will hasten regardless of DRM and that continued use of it only negatively impacts legitimate, paying, customers.
“‘No discernible increase in piracy’ from DRM-free e-books • The Register” http://feedly.com/k/10HZJhI