What most people are missing is that they are not only allowing Microsoft to quote, they are asking for a quote from anyone and will pick the lowest option. They are only specifing that it has to be for a Microsoft solution.
As for restricting to a Microsoft product this is done on almost all RFQ's.
Police departments Restric RFQ to Ford Crown vics because that is the vehicle they have a support network for.
CA CHP restricted their firearms RFQ to S&W 4006 because that is the only weapon they have straind and have repair people for. (This one went thru multiple appeals from other gun vendors, in the end the state mad it case that it was unsafe to support multiple weapon platforms in training and equipment)
The DOD purchases Cisco hardware and its RFQ only support Cisco hardware. This is because all the training is done on Cisco hardware and we can just re-train every time they purchase anothe grup of equipment.
I deal with local govt that request a quote for a VFD, PLC, Contactors, Pilot devices and they specify the devices have to be A-B as this is what they have standardized to. This restricts it to a platform.
They have people trained on a platform Microsoft, If they had to change or support multiple programs just to be fair the cost would be prohibitive.
I know people hate Microsoft but it is a defacto standard in the IT industry since there are so many people trained on how to support.