Jonathan Chait, in a great piece about the McCain camp's dishonesty throughout the campaign:
McCain's proposal of joint town halls was salutary, but it wasn't an act of charity--the obvious purpose was to draw Obama into a forum where McCain excels. Even if McCain did make the offer out of a pure-hearted desire to lift the public discourse, Obama's refusal hardly justifies embarking upon a sustained campaign of slander. McCain's rationale is a bit like saying your rejection from law school forced you to turn to a life of crime.