First, most of these books are old and plentiful, not rare at all. Think "Master Windows Exxcel '95!", not first editions of "Master and Margarita". Libraries routinely destroy plentiful old books that nobody is reading any more. Anything actually rare and valuable is too pricey to hand over.

Second, they HAVE to destroy the books because US copyright law REQUIRES it. They're only allowed to digitize the books if it's considered "transformation" and not "copying". That's only allowed if they destroy the book afterwards.