Steam has updated how demos "appear and behave" within the Steam library.
In a post to the official website, Valve said that with "thousands of playable demos launching on Steam every year, and millions of players trying them out, we've noticed some trends in feedback from both developers and players about the process and functionality. We've put together an update based on that feedback".
Going forward, you'll be able to add demos to your library without having to immediately install them. Just like full games, you can simply click "add to library" and download later on them when you're ready.