They didn’t care at first. The only reason they began destructively scanning books is because they started to care about copyright law:

Anthropic first chose to amass digitized versions of pirated books to avoid what CEO Dario Amodei called “legal/practice/business slog”—the complex licensing negotiations with publishers. But by 2024, Anthropic had become “not so gung ho about” using pirated ebooks “for legal reasons” and needed a safer source.