http://www.ign.com/articles/2013/09/11/steam-family-sharing-announced-enters-beta?utm_campaign=fbposts&utm_source=facebook
Here's the restrictions (which are fair and acts as if the game was a physical game):
-Can share with up to 10 people.
-Games that require 3rd party software (like GFWL), games that require a login (mainly F2P games), and subscription based games cannot be shared.
-Only 1 person can be playing a game at any given time, with the Steam library holder taking priority.
-
So if anyone but MS does it, it's okay?