[phire]

Probably because of the verification.

You can't really verify a shredding, the pile of shredded remains no-longer has a serial number. I assume the cleaning process cryptographically verifies the identify of the disk both before and after the wipe, making it impossible to sneak a drive out.

For those drives which fail the cleaning process, they probably have a complex process with multiple witnesses to ensure it actually gets shredded.