The judge seems to have said it's because they "transformed" the books (destroying them after digitalizing) in the process, that made it legal.

> Ultimately, Judge William Alsup ruled that this destructive scanning operation qualified as fair use—but only because Anthropic had legally purchased the books first, destroyed each print copy after scanning, and kept the digital files internally rather than distributing them. The judge compared the process to “conserv[ing] space” through format conversion and found it transformative. - https://arstechnica.com/ai/2025/06/anthropic-destroyed-milli...