First, the problem isn't inherent to virtual hosting services, you could just as easily get hit by this on a bare-metal site, though the interplay of S3 and Google Docs is an added dimension.
Cutting off all services opens the door for a class of DoS attacks. Simply direct enough traffic at a single account's assets, and you'll knock them offline for a given billing cycle. If the attack is cheap to launch (botnet, URL referral network, etc.) it's a cheap attack. Different entities would have different cut-off and degradation policies.
Better would be to identify the parameters of a specific anomalous traffic pattern, but this can be hard.
A more general solution is to set asset (server side) and client (remote side) caps in tiers. You'd want generous (but not unlimited) rates for legitimate crawlers, your own infrastructure, and major clients. The rest of the Net generally gets a lower service level. Such rules are not trivial to set up, and assistance through AWS or other cloud hosting providers would be very useful.