As such, if you pirated a book and had to pay $3000 for that one instance, I don't think you'd like it if I said you should have paid $30K or $300K instead. If anything, this is analogous to the ridiculous fines people had to pay when pirating music.
(Not that I'm complaining...)
If you pirated a book for personal use the amount of liability wouldn't match a company whose profit could be attributed to pirating the same book. In US copyright law, a copyright infringer could be liable for "any profits of the infringer that are attributable to the infringement" [1] (if the copyright owner elects to recover actual damages and profits instead of statutory damages).
[1] 17 U.S.C. § 504(b), https://www.law.cornell.edu/uscode/text/17/504
Put another way, their revenues wouldn't drop much if they simply hadn't trained on those 99%.
Given the entire global economy is currently bullish on the potential profitability of AI, I dare say they got off incredibly lightly settling for just $3k per book.