My second thought: "I wonder if Amazon use their 'idle' capacity to mine cryptocurrency?"
With respect to my second thought, at their scale, and at the cost they'd be paying for electricity, it could quite possibly be a good hedge.
But most of these markets are very small compared to Bitcoin.
Mine other cryptocurrencies that are profitable, or may become profitable. It is VERY, as in EXTREMELY, profitable to mine large stakes in cryptocurrencies that the market hasn't discovered yet.
Especially ones that aren't even listed on an exchange yet and mining is the only way to earn it.
The Darkcoin (now renamed to the politically non-threatening name DASH) instamine was a notoriously profitable use of GPU clusters on AWS.
Also, if you are paying to mine currencies that don't have an exchange value yet, you are just speculating. The mining isn't profitable at that point because the currency has no value. It's just as likely that you chose to mine a dud. It's exactly the same thing as opening a copper mine at a loss and hoarding the concentrate hoping that it will gain value.
you say that like its a bad thing
it is a very profitable endeavor
But on the other hand, these machines aren't just sitting there powered off when they aren't provisioned. They have to keep them warm so they provision ultra quick.
So maybe there is some level of mining they could accomplish with their "idle" cycles, given the efficiency of the datacenter, the power of their GPUs, and the price of Bitcoin, which would be profitable. I don't know.
But they don't even have to be making a raw profit vs the electricity they are consuming. If, as you say, the spare capacity must be kept warm anyway, then they only need to be profitable vs the delta of electricity consumption between idle and fans blowing 100%.
So whatever capacity is left beyond the fixed and spot market _could_ still be earning them something.
[1] http://cryptomining-blog.com/6090-crypto-coins-to-check-out-...
Edit: fixed spelling.