Re: So, three years on ...
"Oh, sorry. It isn't supported on Azure."
Yet. Obviously, the winning cloud provider will have a scheme whereby third-party suppliers to the DoD can pay them lots of cash to make their product lines compatible with the DoD's preferred cloud.
Ten years on, the cloud provider has the additional advantage during the bidding for renewal that almost every third party's products are compatible with their cloud but (quite possibly) not the opposition (or, at least, not demonstrably so). So they get the renewal. Repeat ad infinitum.
Who pays? Well, the successful cloud provider makes a shed-load of dosh from the third party suppliers. Those suppliers pass on the costs (now anonymised) to the DoD, which forks out using tax-payers cash. So, er, I guess it is US tax-payers who pay.
Initially. Of course, the next stage is for it to become a requirement all across NATO.
Free market economics: it's great. Someone ought to tell the US government about it. At the moment, they appear to believe in the magic money tree.