Because they can. On Monday, Facebook released a video that shows how their new sponsored stories program works. Now when you update a page, like, check-in or interact with an application and mention a customer who has paid for this service, it will appear in your newsfeed as per normal, as well as in the right hand advertisement column. Facebook has said that this advertisement you have provided them will only appear to those who you’ve authorized in your privacy settings.
If you’re doing any of these things you should be aware of this as Facebook is not a volunteer organization, they make money by selling the information you willingly provide them.
If you do not want to participate in this program, the recommended steps are: