They're really free. They aren't the only ones doing them, either. EA's Origin store used to give away a game every month or two. GOG often gives away a game at the start of their various sales. Steam will give away games on rare occasion, too, though they more often give a "free weekend" where you can play a game through the weekend for free. If you want to keep a "free weekend" game, though, you'll need to buy it. (Of course, if the game is short and you play it a lot, you may be able to finish it in a single weekend!)