What do protected tweets do that a Facebook status update wouldn't? Public tweets have added value over Facebook statuses for some people, but I don't see where protected tweets would.

Nothing, but I was responding to the article stating that Twitter is for broadcasting whereas Facebook keeps updates from being public.

Defaults matter. Twitter is public by default and the whole ecosystem is based on this assumption, the private setting is just an afterthought. Sure, you can tweet privately, but then you lose most of the features, and I wouldn't rely on security of that.