Apparently hardware depreciation is the dominant cost rather that operational cost (electricity etc), and this is occuring at a fixed rate per the planned replacement lifetime.

So, the cost to provide the service is essentially fixed regardless of load, but the revenue they are generating is variable. In practice most GPU's are going to be capacity-maxxed since the providers sell cheap batch APIs that they queue to keep the machine loaded. They'd be losing money over a given time interval if revenue generated during that interval wasn't greater than depreciation (etc), but it seems that will rarely happen.