Comment 7 for bug 686133

I don't know the official reason, but for Unity it makes sense because the windows aren't going down, they're going left.

As for the consistency with GNOME Shell, in later mockups it seems they might be getting rid of Maximize as well. So Maximize/Unmaximize would be open for Unity. Granted that is another 'bug' if we chose to change that icon to reflect the global menu behavior.

I've attached a quick mockup of how the minimize button would look on the left as well as centered (since the minimize button is currently centered)
It also contains an icon for unmaximize, but again that would be another bug.