When the President doesn't these types of events, the meeting is behind closed doors then they'll both step out and answer questions.
I think everyone remembers the number of meetings with Tony Blair when you could see Tony wince in pain at Bush fumbling throught the questions after his articulate answer
The Bush Admin has always treated the Press as an enemy and has gone to great lengths to limit their exposure. The McCain campaign is attempting to further this and feed off of the "liberal media bias" to rally the base.
Given this, they really have no motivation to let the Press report on campaign activites. Their strategy is clearly to keep Palin in a bubble and hope the voters see her as a right wing pin-up rather than someone who's been vetted on the public stage.
-spence