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They probably get you in bandwidth fees over X amount. It would cost pennies for a small scale virtual server with big numbers as the hardware is shared, it would spend most of the time not doing anything. They could set up a machine and oversell a tier like that and make it all back with profit on their first bill.
I bet they run these free accounts on their test infrastructure, not production. What they get from it is real-world user testing of changes to their infrastructure, similar to how Microsoft uses its Windows Home versions for testing new updates.