You think Obama voted for FISA because he woke up one day and thought it was a good idea? What about NAFTA, you think he had an epiphany? Oil drilling? Faith based initiatives?
Obama is 'reinterpreting' his positions to appeal to a broader audience, those he can't reach with his rhetorical skill. Politics.
McCain is trying to tear down a much more charismatic opponent while talking about his resume (which is, you know, damn impressive). Politics.
It's how the game is played. Many things I don't like about McCain, the fact that he is playing to win is not one of them. (He is up 4 in Gallup since the DNCC started so this is clearly working for him.)