Their reasoning (and I'm not supporting it) is probably that with the current DRM stripping tools, you basically have to have an authorized copy (tailored for you or your reader) before you can even remove the DRM. I can't remove the DRM from
your ebooks without personal information about you and/or using your physical computer. Basically, the only one who can remove the DRM is the one who went through the proper channels to get an authorized copy (for the most part).
Right now, there's two "proper channels:" buying (licensing... whatever) the ebook from a store. Or borrowing it from a library. Guess which scenario the publisher would rather you used??
People who spend their hard earned money on an ebook are less likely to mass share/upload their DeDRM'ed copy than someone who checks out a free library book and strips the DRM. The libraries have the potential to be a wholesale outlet for bulk pirates... and they don't even have to spend a penny.
Library books are the one situation where I support DRM. I wish the current tools
didn't work on any library books, but when they use the exact same DRM on library books that's on retail books... what's to be done? You can't really unring the bell, ya know?