As the title suggests I haven't been able to come up with any particularly compelling justifications. Perhaps being a non-founder comes in to play, but more specifically the following job ad piqued my interest:

http://news.ycombinator.com/item?id=3481790

From the ad they say that their company has a user to employee ratio of 10.5 million. Bringing on an additional hire(s?) would lower the user/employee ratio to about 7.5 million. So the question is then, if you have that many users already, what is the point of an anonymous job posting? If you are already that big then surely your competitors know who you are. What value do you get from all of this seemingly arbitrary secrecy?