Taking the hard-work of the open-source community and making money off it without any acknowledgement or payment to the origin of the product is just immoral, not that it matters much, but it does a great job of making you come off as an asshole.
Like was said, Legally Ok. I don't think they're bitching because Microsoft using it, but rather, because Microsoft is taking their hard work and making a profit off it without doing anything in return. In Polish, we have a word for this kind of behavior: Hamstwo (Haamstvo-phonetically).
No one's bitching from a legal standpoint, they understand Microsoft is allowed to do this, they're just annoyed at the disregard these people are showing.