If Office and Azure are operate by two companies owned by Microsoft instead of MS directly then MS the one operating Office would have to pay the one operating Azure for the hosting.
AWS is not a separate company and even if they were, there is nothing stopping one company from negotiating with another company for special negotiated rates.
You don’t think Netflix is paying rack rates for AWS do you?
no AWS _is_ a different company then Twitch, Amazone owns Twitch but Twitch isn't Amazone.
and yes you can always negotiate rates but it has to be in some "reasonable boundaries" so Twitch still has to pay Amazone and Amazone can't just set the price to just the cost Amazone has, even less so below.
Should Microsoft also not be allowed to host online Office on Azure?