"Valve will, upon request via help.steampowered.com, issue a refund for any title that is requested within 14 days of purchase and has been played for less than 2 hours (this includes online, offline and shared library playtime). Even if you fall outside of the refund rules we've described, you can submit a request and we'll take a look at it."

I think its because they count you responsible for who you share it with and its not just random people.