Because destroying the book makes it possibly inaccessible permanently because we have no idea how long anthropic plans on storing the digital copy, if at all. A lot of people assume they would for future training, but you don't know that.

If they at least didn't destroy the book, someone could purchase it when anthropic eventually goes belly up. Hopefully someone will at least be able to purchase their digital scan and hopefully the scan is of decent quality and clearly indicates the provenance of the text.