The price is more or less an indicator. If they can shred a $1000 book, kudos to them or their budget.

The idea they're simply shredding human history is too one-sided and alarmist. There is a huge long tail of rare-ish old and scrappy books out there that are definitely not the last copy of anything.

That said, I'm pretty sure a very small % of these books fall into the mistake category. But life!