Where do you get that? I've never heard of Let's Players getting free games specifically for Let's Plays.
For example a lot of lets players got copies of Kingdoms of Amalur or FFXIII to play it before the release date on either youtube or twitch.TV. Total biscuit for example got this quite often already. 2K games right now gave some copies of CIV 5 brave new world to lets players if you want a recent example. More and more companies realize that this is huge free PR if the game is good of course.
Are you going to actually cite anything? Provide links? Show me evidence that leads me to believe you're telling the truth and not just pulling things out of your ass?
Game Grumps just got Civ 5 brave new world for their 'Let's play' show. http://www.youtube.com/watch?v=wLLJ4EZwaNc