That's cool and all, but a Windows client would make a lot more business sense. There are already a couple of nice git clients for Mac. There aren't for Windows. And I assume github is targeting SMB with its paid offerings. Unless you're a design shop or a hip startup, odds are you're not running OS X. Based on my unscientific data, most companies have the devs working on Windows (or occasionally Linux).
They are the latter. Welcome to the San Francisco bubble, where you think everyone uses iPhones and Macs, because in your world everyone does. It's the same reason Twitter has a Mac app but not a Windows app. They build these things for themselves (and arguably they should).