We used to have that as a feature of JFBConnect until about the v4.0 release. At that time, Facebook released a specific Platform Policy update (their usage guidelines) that forbid automatically posting when a user registers or logs into your site. After that release, we removed the feature.
Facebook, simply, doesn't want 'low quality' content shared on the user's behalf. They want higher quality updates (like Open Graph Actions of what the user is actually 'doing' on your site) or intentional updates like Comments, Likes and Shares. Things like simply logging in just don't have enough context for Facebook to want a user's stream to be filled with them.
I hope that helps answer your question!
Alex
Facebook login: automated post on the wall
