Second: My understanding is that the licensing models of Microsoft become hideously expensive as you scale up. Why waste money?
Third: Ethics about proprietary software; I think it's considerably more ethical to use and advocate libre software.
edit: Fourth: Windows is harder to get @#$^ done in, IME. I've generally lived in the F/OSS ecosystem as much as I could for the last 5+ years; whenever I want to do something that I'd do in F/OSS in Windows, it seems like "everything is hard". The complexity required to do anything programmatic is significant compared to using Linux/OSX, even if you just measure in keystrokes/clicks. This is, I'm sure, partially due to my inexperience, but I'm reasonably sure that it's a combination of Big Enterprise Design Pattern thinking along with Cater To Non-Hacker thinking.