Do you expect Nintendo to make their own office software and infrastructure? No, no you don't. MS is the most trusted for a wide majority of reasons, and tend to be the most consistent throughout their ecosystem. It just business logic. Hell they were probably using it even before MS even came out with the Xbox.
Also companies do this shit all the time, they profit off each other. A good example being that MS profits off of Android devices because of a file format that SD cards use which is a microsoft owned format which requires licensing to use. Another good example are hardware parts in devices made by competitors, such as Apple iPhones using Samsung to produce their screens and various other parts. That is the corporate world, son. It's not as black and white as the fanboys make it out to be.
