Now we know where the wiki leaks came from. 
Basically its a trust issue. Shredded and crushed disks can't be read. Wiped disks end up on the "black market" not wiped ! Especially when some one tries to sell the idea of wiped disks or "we'll" wipe them for you and don't!
Dead/Crushed and shredded disks don't haunt governments!
yeah, because its so hard to write a simple code yourself that wipes?
or simply use different systems, one for wiping, other for verifying its wiped?
heck, you could fill it with bogus data.
shredding them seems wasteful. besides, its probably not environment friendly.