So if you want to do both unixy world and games why not Mac?.
1) Some people like Linux more than either of the proprietary OSes. This might be because they can configure Linux more, or because it's free, or because it's ideologically free, or because their friend told them to run it, or any of a thousand other reasons.
2) Why not? Many indie developers have already made Linux-compatible games that are also on steam. For instance, most of the Humble Indie bundles have had a requirement of running on Linux, and most of those games also provided steam keys.
3) Other people aren't a Apple shills/trolls?