Companies like Microsoft do have a reputation to maintain. Remember the howls of derision when Microsoft tried (and failed) to move Hotmail from BSD to NT? Their critics had a field day. It's reasonable for MS to want to only be associated with MS gear.
Think about the reverse... kernel.org running off a windows machine. A lot of people would see something wrong with that.
You can see why MS marketing people don't want to have to answer phone calls from the press saying "If you guys are so good, why are you using someone elses product? Maybe we should too?"
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Jeremy
I didn't believe in evil until I dated it.