Well, if you are hacked, you're not liable for the fraudulent charges anyway (in the U.S.... mileage may vary elsewhere). Your CC company will charge it back, in the unlikely event that Amazon didn't take care of it on their own initiative (which they almost certainly would... as another poster noted, they have a good reputation in that area).

I interpreted "doing something silly" as "firing up 10,000 instances and forgetting to shut them back down", rather than fraud.