That's an interesting idea. I imagine they don't do DoD wipes of their disks because they are probably multi-tenant, right? Can a guest operating system do forensic analysis on the host disk?

AWS wipes disks before reusing them (loosely speaking--an EBS volume doesn't correspond 1:1 to a physical disk). Forensic analysis of overwritten data requires disassembling the physical disk, so no, it's not possible.

Disks are wiped immediately before are made available as a new volume. If your policies require a certain level of wipe, you should wipe the disk before releasing it.