Can you imagine walking up to a guy on the street and saying something like "Hey there. If you tell me the names and contact information for all of your friends, the conversations you've had with them, the type of products you like, your occupation and lots of other demographic information? I'll let you play this little game with bottle caps" Yet to many technologists working on the internet this seems like a perfect…
Edit: this is not supposed to be a direct analogy (obviously Facebook does not have your medical records yet, afaik). Games want to have the ability to find out who is your friend and to post on your behalf, it's not absolutely necessary but millions of people understand it's the price they pay for a free game.