They don't have to destroy the books to scan them. There are techniques for doing it non-destructively, but they're slower and more expensive. It's cheaper to cut the spines off and drop the stack of papers on a flatbed scanner with a paper feeder.
Destroying the original makes it easier to justify that scanning the books is fair use, because you haven't increased the number of copies. See page 16 of https://copyrightalliance.org/wp-content/uploads/2025/06/Bar... : "This use was even more clearly transformative than those in Texaco, Google, and Sony Betamax (where the number of copies went up by at least one)"
Destroying the original makes it easier to justify that scanning the books is fair use, because you haven't increased the number of copies. See page 16 of https://copyrightalliance.org/wp-content/uploads/2025/06/Bar... : "This use was even more clearly transformative than those in Texaco, Google, and Sony Betamax (where the number of copies went up by at least one)"