I take that for granted too, but it does seem like a dilemma for AWS. If instances rarely terminate, people psychologically rely upon them to remain running and end up getting careless. But it's distasteful to arbitrarily end people's instances every 7-10 days just to lower their expectations.
i think there are EC2 images now with S3 backup software pre-installed... I would think it's not that hard to install that to an existing image either...