1. McCain does better when people ask him tough questions. If he wanted to stack an audience to favor him, it would be sprinkled with some radical Leftists -- which would only make him look better by comparison.
2. Based on his absense yesterday, Obama clearly does not want to be involved in these town hall meetings. So it's in his best interest to try to find a straw man to knock down.
3. As evidenced here, here, here, and here, McCain is famous for his willingness to take tough questions.
... Obama, on the other hand, couldn't handle more than eight questions in a press conference about Tony Rezko. As you'll recall, McCain took 36 questions about the bogus NYT lobbyist affair story ...