I've looked and looked and cannot find a reason for hiding login and registration forms from a user after that user has logged in. Aside from the obvious, such as removing the login form because it is unnecessary to log in again, are there security reasons? Are there issues with sessions if allowing the login form to continue to be exposed after logging in and the user logs in again? Any information is greatly appreciated.
To your pointed technical question, there are no technical issues with allowing the form to continue to exist client-side. It's always a click or two away in any case.
If asked over on the UX SE, I'm sure it would come up that it's a waste of space. Almost universally, interfaces should be individually optimized for the current situation. Even Google opts to offer logging in with additional accounts, but it is suitably tucked a couple clicks deep, due to the comparitive rarity of that action.
But as I'm sure you'd hear in first over on Information Security SE though, there's the big reason: filling a window with personalized things puts each user in the mindset that this session is for them, and quietly encourages people to log out when done. If a UI in any way looks "ready for the next person," it's implicitly encouraging those unconcerned, lazy, or naïve to just walk away—fine at home, horrible for a library, community center, school, conference center, and so on.