Nonsense. A business ought to understand the risk before accepting the deal. If they underestimated the risks, they are punished with loss of revenue; if they correctly evaluated the risks and priced the service accordingly, they stand for a profit. A business' duty is to honor the contracts they sign. Of course, this is in the ideal world, where parties signing a contract have a fair balance of power; in the real world, amazon dictates the rules, and is in a position to craft some really vague ones at that.
The way I see it, amazon isn't simply breaking the contract over the direct cost of the demanded service like GGP supposes, mostly because the added cost caused by the "unexpected" traffic of cablegate is only marginal to amazon. Also, it is my understanding that amazon charges for bandwidth spent almost linearly, so all this "unexpected" traffic is being charged to Assange anyway. I believe they think there's a indirect cost in upsetting a potentially large client such as the US govt. In other words, they weighed the loss of reputation and money for booting wikileaks against the future loss of opportunities with USG and decided for booting wikileaks.