I don't think they're going to enforce a "you went way over the storage other people tend to use" cap, the way ISPs tend to. S3 stores petabytes of stuff; they can eat your 50GB-per-day wastefulness without breaking a sweat.

I think it's more to cover the case where you figure out a way to build a business that stores data in this, instead of in S3. Especially the case where that business is, itself, a primarily data storage or backup business, so you're directly living off the margin between "capital-cost storage for you" and "linear-revenue storage sold to customers." Amazon does not want to kill S3's business model, just yet.