You'd need to integrate it with each VM running, possibly just be sending it the equivalent of a shutdown command from the console, so that it understood that the disk was going away in X seconds, and to shutdown any databases immediately, flush all cache, unmount filesystems, and shutdown itself.
It wouldn't be massively difficult, but not as simple as simply shutting down EBS.
On the other hand, an fsck to recover the filesystem probably causes even more traffic.
My guess is that this functionality is missing for historical reasons -- the original S3-backed instances didn't have any concept of powering off an instance.
(generators and UPS might be cheaper, especially since all facilities have them, than getting all users to be good at handling failure gracefully. It's ok for an in-house datacenter doing batch processing, or with great developers, like Google, but probably not for a generalized IaaS)