I think it's less about selling the machines, and more to get people using SteamOS. Like how Valve used Half-Life 2 as a trojan horse for Steam back in the day.
Maybe HL3 development got restarted or something, like it happened to HL2 It would get a lot of steamOS installs on day one...
Not to mention it could just be a code for the game that comes with the Steam Machine. They don't release it for everyone until the month after the Steam Machine is released. Would be genius.