I wonder why they do it for some events but not all?
Tim Cook said this in his email to employees following the last event -
"We made these announcements in Town Hall here in Cupertino this morning. It is a small venue which limits our ability to show the keynote live across campus and to other locations as we like to do. The event is now available to stream on apple.com, so I hope you will take time to watch it."
I think in the case of Town Hall based events, this is the answer to that question.