It's a bit different when a corporation open sources their product because it brings them more business value than a corporation actually contributes to not their open source projects. Does MS do that?
Are there any examples where a company contributes to Open Source software, without any (direct or indirect) benefit to themselves? What difference does it make anyway?
My impression, having spoken to people from Microsoft involved in OSS, is that they are pretty much entirely driven by what will make them the most profit. Sometimes that means making software "open source" and accepting contributions from the public (free labour!), other times that means keeping their software proprietary. They don't seem to care about ethics, or software freedom. It's a shame, but Microsoft aren't the only ones with that mentality. Big businesses are, more often than not, primarily motivated by profit.