It's legit. They have you install it so they know accounts aren't hoarding it from the mobile app. Once you own it, you own it, so you should be able to uninstall it and throw it wherever you want.
http://www.l4d.com/blog/post.php?id=12152
http://www.l4d.com/blog/post.php?id=12152
To claim your L4D2 gift, all you need to do is install it.
If you don’t install, it goes back to its regular price and you miss out! The zombie apocalypse is more fun with friends so make sure to tell all your mates to install it as well.